Enable to build custom Linux image

All Buildroot, BusyBox, and Linux configuration files are saved in the
assets directory to allow someone to build on their own. The
mk/external.mk functions has been updated to enable downloading via
'git clone' and verifying the downloaded files excluding the
git-related files. The Linux kernel image and Buildroot version are
controlled in mk/external.mk, when the LINUX_VERSION or
BUILDROOT_VERSION changes, the new Linux kernel image and rootfs.cpio
are built and pushed to the rv32emu-prebuilt repository, the user ELF
executables and system image are identified by suffix 'ELF' and
'Linux-Image' at release of rv32emu-prebuilt repository. Additionally,
a 'build-linux-image' make target has been introduced to simplify the
cloning and automatic building process.

Author: ChinYikMing

.github/workflows/build-artifact.yml

@@ -27,6 +27,12 @@ jobs:
             tests/quake/**
             tests/scimark2/**
             tests/*.c
+      - name: Test file change of Linux image
+        id: test-linux-image-version-change
+        uses: tj-actions/changed-files@v45
+        with:
+          files: |
+            mk/external.mk
       - name: Set alias
         id: has_changed_files
         run: |
@@ -35,8 +41,60 @@ jobs:
           else
             echo "has_changed_files=false" >> $GITHUB_OUTPUT
           fi
+          if [[ ${{ steps.test-linux-image-version-change.outputs.any_modified }} == true ]]; then
+            # Determine if the changes are from Buildroot or the Linux version (The Linux might have several patches, so also need to check the SHA value)
+            echo -n $(git --no-pager diff HEAD^ HEAD | grep -e "+BUILDROOT_VERSION" -e "+LINUX_VERSION" -e "+LINUX_DATA_SHA1") >> linux-image-version-change
+            if [[ -s linux-image-version-change ]]; then
+                echo "has_changed_linux_image_version=true" >> $GITHUB_OUTPUT
+            else
+                echo "has_changed_linux_image_version=false" >> $GITHUB_OUTPUT
+            fi
+          else
+            echo "has_changed_linux_image_version=false" >> $GITHUB_OUTPUT
+          fi
     outputs:
       has_changed_files: ${{ steps.has_changed_files.outputs.has_changed_files }}
+      has_changed_linux_image_version: ${{ steps.has_changed_files.outputs.has_changed_linux_image_version }}
+
+  build-linux-image-artifact:
+    needs: [detect-file-change]
+    if: ${{ needs.detect-file-change.outputs.has_changed_linux_image_version == 'true' || github.event_name == 'workflow_dispatch' }}
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          submodules: 'true'
+      - name: Install dependencies
+        run: |
+          sudo apt-get update -q -y
+          sudo apt-get upgrade -q -y
+          sudo apt-get install -q -y build-essential git
+      - name: Build Linux image
+        run: |
+          make build-linux-image
+          make artifact ENABLE_PREBUILT=0 ENABLE_SYSTEM=1
+          mkdir -p /tmp/rv32emu-linux-image-prebuilt/linux-image
+          mv build/linux-image/Image /tmp/rv32emu-linux-image-prebuilt/linux-image
+          mv build/linux-image/rootfs.cpio /tmp/rv32emu-linux-image-prebuilt/linux-image
+          mv build/sha1sum-linux-image /tmp
+      - name: Create tarball
+        run: |
+          cd /tmp
+          tar -zcvf rv32emu-linux-image-prebuilt.tar.gz rv32emu-linux-image-prebuilt
+      - name: Create GitHub Release
+        env:
+          GH_TOKEN: ${{ secrets.RV32EMU_PREBUILT_TOKEN }}
+        run: |
+          RELEASE_TAG=$(date +'%Y.%m.%d'-Linux-Image)
+          cd /tmp
+          gh release create $RELEASE_TAG \
+            --repo sysprog21/rv32emu-prebuilt \
+            --title "$RELEASE_TAG""-nightly"
+          gh release upload $RELEASE_TAG \
+            rv32emu-linux-image-prebuilt.tar.gz \
+            sha1sum-linux-image \
+            --repo sysprog21/rv32emu-prebuilt
 
   build-artifact:
     needs: [detect-file-change]
@@ -83,7 +141,7 @@ jobs:
         env:
           GH_TOKEN: ${{ secrets.RV32EMU_PREBUILT_TOKEN }}
         run: |
-          RELEASE_TAG=$(date +'%Y.%m.%d')
+          RELEASE_TAG=$(date +'%Y.%m.%d'-ELF)
           cd /tmp
           gh release create $RELEASE_TAG \
             --repo sysprog21/rv32emu-prebuilt \

assets/system/configs/buildroot.config

@@ -0,0 +1,47 @@
+BR2_riscv=y
+BR2_ARCH="riscv32"
+BR2_NORMALIZED_ARCH="riscv"
+BR2_ENDIAN="LITTLE"
+BR2_GCC_TARGET_ABI="ilp32"
+BR2_READELF_ARCH_NAME="RISC-V"
+BR2_RISCV_ISA_RVI=y
+BR2_RISCV_ISA_RVM=y
+BR2_RISCV_ISA_RVA=y
+# BR2_riscv_g is not set
+BR2_riscv_custom=y
+BR2_RISCV_ISA_CUSTOM_RVM=y
+BR2_RISCV_ISA_CUSTOM_RVA=y
+# BR2_RISCV_ISA_CUSTOM_RVF is not set
+# BR2_RISCV_ISA_CUSTOM_RVC is not set
+BR2_RISCV_32=y
+# BR2_RISCV_64 is not set
+BR2_RISCV_ABI_ILP32=y
+BR2_BINFMT_ELF=y
+BR2_TOOLCHAIN_BUILDROOT_VENDOR="buildroot"
+BR2_TOOLCHAIN_BUILDROOT_GLIBC=y
+BR2_TOOLCHAIN_BUILDROOT_LIBC="glibc"
+# BR2_TOOLCHAIN_BUILDROOT_CXX is not set
+BR2_KERNEL_HEADERS_6_1=y
+BR2_BINUTILS_VERSION_2_42_X=y
+# BR2_BINUTILS_GPROFNG is not set
+BR2_GCC_VERSION_14_X=y
+BR2_EXTRA_GCC_CONFIG_OPTIONS="--enable-softfloat"
+BR2_TOOLCHAIN_HEADERS_AT_LEAST="6.1"
+BR2_TOOLCHAIN_GCC_AT_LEAST_14=y
+BR2_TOOLCHAIN_GCC_AT_LEAST="14"
+BR2_SSP_NONE=y
+# BR2_SSP_REGULAR is not set
+# BR2_SSP_STRONG is not set
+BR2_RELRO_NONE=y
+# BR2_RELRO_PARTIAL is not set
+# BR2_RELRO_FULL is not set
+BR2_FORTIFY_SOURCE_1=y
+# BR2_PACKAGE_URANDOM_SCRIPTS is not set
+BR2_TARGET_ROOTFS_CPIO=y
+BR2_TARGET_ROOTFS_CPIO_FULL=y
+# BR2_TARGET_ROOTFS_CPIO_DRACUT is not set
+BR2_TARGET_ROOTFS_CPIO_NONE=y
+# BR2_TARGET_ROOTFS_TAR is not set
+BR2_PACKAGE_BUSYBOX_CONFIG="busybox.config"
+BR2_PACKAGE_COREMARK=y
+BR2_PACKAGE_DHRYSTONE=y