add initial support for feather 52833

- add linker and startup for 833
- add s140 7.3.0 header API
- add build.uf2_family, update platform.txt
- update makeboards.py to generate 833
- add 833 variant based on 840

Author: hathach

boards.txt

@@ -0,0 +1,38 @@
+/* Linker script to configure memory regions. */
+
+SEARCH_DIR(.)
+GROUP(-lgcc -lc -lnosys)
+
+MEMORY
+{
+  FLASH (rx)     : ORIGIN = 0x27000, LENGTH = 0x6D000 - 0x27000
+
+  /* SRAM required by Softdevice depend on
+   * - Attribute Table Size (Number of Services and Characteristics)
+   * - Vendor UUID count
+   * - Max ATT MTU
+   * - Concurrent connection peripheral + central + secure links
+   * - Event Len, HVN queue, Write CMD queue
+   */
+  RAM (rwx) :  ORIGIN = 0x20006000, LENGTH = 0x20040000 - 0x20006000
+}
+
+SECTIONS
+{
+  . = ALIGN(4);
+  .svc_data :
+  {
+    PROVIDE(__start_svc_data = .);
+    KEEP(*(.svc_data))
+    PROVIDE(__stop_svc_data = .);
+  } > RAM
+
+  .fs_data :
+  {
+    PROVIDE(__start_fs_data = .);
+    KEEP(*(.fs_data))
+    PROVIDE(__stop_fs_data = .);
+  } > RAM
+} INSERT AFTER .data;
+
+INCLUDE "nrf52_common.ld"