Merge pull request #10323 from maribu/stm32f103c8_rom_hack

boards: Enable STM32F103C8 ROM hack via env var

boards/bluepill/doc.txt

@@ -61,12 +61,12 @@ you can flash the device with:
 
 To make use of the entire 128 KB flash, compile your application with:
 
-    $ make BOARD=bluepill CPU_MODEL=stm32f103cb
+    $ make STM32F103C8_FLASH_HACK=1 BOARD=bluepill
 
 This sets the `CPU_MODEL` make variable to `stm32f103cb`, the default
 value is `stm32f103c8`. These two CPU models basically only have one
 major difference, the latter has 128 KB flash while the former has 64
-KB.
+KB. You may also set `STM32F103C8_FLASH_HACK` as environment variable.
 
 If you want to flash a binary compiled this way you either need to
 figure out how to adjust the OpenOCD configuration to make it use 128 KB

boards/common/stm32f103c8/Makefile.include

 ## the cpu to build for
 export CPU = stm32f1
-export CPU_MODEL = stm32f103c8
+STM32F103C8_FLASH_HACK ?= 0
+ifneq ($(STM32F103C8_FLASH_HACK),0)
+  export CPU_MODEL = stm32f103cb
+else
+  export CPU_MODEL = stm32f103c8
+endif
 
 INCLUDES += -I$(RIOTBOARD)/common/stm32f103c8/include