Add support for building flashable kernel zips

Choose device from lunch menu then enter make bootzip, builds flashable boot.img with updated source ramdisk

Change-Id: I07820d5e5e48d5b973901ed2bb3f330744c6498c
build: Set ccache base to build top

This enables ccache sharing between different build tree paths.

Change-Id: I99636bdd2779a6ccf0649f027e35cf67df44b6ae
Don't set CCACHE_BASEDIR if its already set

 * enable use of shared ccache

Change-Id: I0414eeff65121ed482c80311057ae4753f172da7