Makefile: add list-defconfig to the general help

When listing defconfig files was moved to its own make target, it was
not added to the general help text.

However, this is a very important topic, so list it.

[Peter: drop ':' character]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>