*.mk: Use empty recipes for .PHONY targets

*.mk: Use empty recipes for .PHONY targets

The @: dummy recipe was there to avoid having rules without recipes.
Rules without recipes can be confusing, as the reader can't know if a
recipe is being defined somewhere else.  Also, implicit rules might
apply (in general, but we disable all implicit stuff).

However, @: is also problematic, since make really needs to run that
command, which is unnecessary overhead.  But okay, :(1) (really called
true(1), but wouldn't it be a nice man page name?) that's not meaningful
overhead.

And the bigger problem: having a recipe hides the usual:

	make: Nothing to be done for 'all'.

We already disable it by using .SILENCE:, but the message can be seen
when using V=1 (after this patch; prior to this patch, it was impossible
to find that message).  It is good to be able to know that make(1)
says there's nothing to be done, as a confirmation.

Signed-off-by: Alejandro Colomar <alx@kernel.org>