Makefile: add alldefconfig target

It is used by Kconfig's merge_config.sh.

No alldefpackageconfig is added, since it's rather pointless: it would
only enable busybox.

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