Makefile: Create an empty .config if needed

default: build/system.elf

# When running ./mp arch=... app=..., a .config file is not needed.

# Create an empty one if it does not exist yet.

.config:

	touch .config

include/config.h: .config

	${QUIET}test -z "${app}" && awk -f script/conf2h.awk .config > include/config.h || : > include/config.h