- Nucleo-f091rc can become unflashable when hardfaults occure.
To make sure flashing succeeds `connect_assert_srst` is called
before connecting to flash threw openocd.
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.