- Move make/local.mk to the top of the Makefile, so it can
set any variables
- Move target and group definitions to make/targets.mk
- Move settings related rules to make/settings.mk
- Move build-stamp rules to make/stamp.mk
- Define STM32 flash offset in a variable in make/mcu/STM32.mk
- Add make/openocd.mk with openocd-run and openocd-flash targets
- Add make/gdb.mk with gdb-openocd target
