Makefile: split off 'list-defconfigs' from 'help'

As discussed in the FOSDEM2015 BR developer meeting, the output of
'make help' is too long for comfortable reading. To shorten it, split
off the list of defconfigs in a new target, 'list-defconfigs'.

Declare the new target as phony.

Add 'list-defconfigs' to the documentation.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>