Makefile.include: flash: do not peek into MAKECMDGOALS.

When flash-only was introduced (in #8373), the `flash` rule was
made conditionally dependent on `all` by looking for `flash-only`
in MAKECMDGOALS. This was done to avoid code duplication.

There's a cleaner way, by using canned recipes. When we upgrade the
requirements to gnu make 4, the flash recipe can be defined as ?=.