Makefile: printvars: don't print anything when VARS is not set

Makefile: printvars: don't print anything when VARS is not set

Using 'make printvars' for printing all variables is not very useful.
E.g. all macros will output some bogus value. In addition, the same can
be achieved with 'make -p'.

We can simply remove the condition on $(VARS). If VARS is not set, the
filter expression will be empty which matches nothing, so nothing is
printed.

Note that the old behaviour can still be achieved with:
make printvars VARS=%

Update the 'make help' text to match the new behaviour.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>