Previously trying `make debug` on these boards would result in
Error: Unable to locate OpenOCD configuration file
(/home/benpicco/dev/RIOT/boards/saml10-xpro/dist/openocd.cfg)
Add a common `openocd.cfg` to fix this.
cpu/$(CPU)/Makefile.features and cpu/$(CPU)/Makefile.dep are
automatically included
Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.