mms: introduce a shortcut to quickly rebuild kernel/boot.img

Add a new shortcut to short circuit the long build process that walks
the entire tree collecting all Android.mks. The resulting dependency
list is sufficient to rebuild the kernel, copy product files and
repack the system image with updated modules.

This shortcut allows Kernel devs to rebuild just the boot.img and
kernel modules very quickly (20s vs 3min).

Change-Id: Ie0a69f241ea7b920859ff1e02c3542b79952462c