boards/im880b: refactor code

Using the common STM32 board makefile provides all supported
programmers for STM32 boards. This allows e.g. to flash with
stm32flash via `make BOARD=im880b PROGRAMMER=stm32flash flash`.

boards/im880b/Makefile.include

@@ -8,9 +8,4 @@ PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.SLAB_USBtoUART*)))
 CFLAGS+=-DSX127X_TX_SWITCH
 CFLAGS+=-DSX127X_RX_SWITCH
 
-# this board uses openocd
-PROGRAMMER ?= openocd
-# openocd programmer is supported
-PROGRAMMERS_SUPPORTED += openocd
-
-OPENOCD_DEBUG_ADAPTER ?= stlink
+include $(RIOTMAKE)/boards/stm32.inc.mk