busybox

Support configuring Busybox from _defconfig files

The existing test .config files are moved to configs/ and renamed
to *_defconfig. 'make xyz_defconfig' will enable the
configuration in that specific file.

Signed-off-by: Daniel Fandrich <dan@coneharvesters.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>