config: make it possible to specify which config file to use for 'make defconfig'

So that this works:

 % make defconfig BR2_DEFCONFIG=~/buildroot-defconfig

Right now we have to do:

 % cp ~/buildroot-defconfig configs/buildroot_defconfig
 % make buildroot_defconfig

That would dirty the buildroot tree and requires it to be writable.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>