Add platform 'Feather_M4'

Author: Dennis-van-Gils

cpp_Arduino_wave_generator/platformio.ini

@@ -7,8 +7,17 @@
 ;
 ; Please visit documentation for the other options and examples
 ; https://docs.platformio.org/page/projectconf.html
+[platformio]
+env_default = adafruit_feather_m4
 
 [env:mzeropro]
 platform = atmelsam
 board = mzeropro
 framework = arduino
+
+[env:adafruit_feather_m4]
+platform = atmelsam
+board = adafruit_feather_m4
+framework = arduino
+upload_protocol = sam-ba
+lib_ldf_mode = chain+