Fix custom recovery build

Taken from:
https://github.com/omnirom/android_build/commit/c16e7487526ffe26a1f76d0db4987d8c37f8b583
https://github.com/omnirom/android_build/commit/300ffcd78a91e3cb3d5abeadca40b2731b5a23a8
https://github.com/omnirom/android_build/commit/c6b0592ec526a690ca7003664474ff1e8c96fc06
https://github.com/omnirom/android_build/commit/1b8ca81a0cc738c5ddc27dfdb0c1dc468dc4196c

Also add kernel.mk for kernel build