Add support for building flashable kernel zips

eg: make -j8 bootzip

squash of:

https://github.com/LiquidSmooth/android_build/commit/c987e7afe6c1a585bd8147c6cc626f95e41e97dc
https://github.com/LiquidSmooth/android_build/commit/3e0ce150112aecdca652b353a2f94a927782529a
https://github.com/LiquidSmooth/android_build/commit/01ec9078f0b3a836ef55af52fab3693dc1d43e83
https://github.com/LiquidSmooth/android_build/commit/ae9e722342f4ca56d251cfdae6ba3892fa7e207c
https://github.com/LiquidSmooth/android_build/commit/03e0cfdca700aac86df859acd26b5225be915ce3

credit to the original authors

Change-Id: Iffb3e66efd260b3af77654062c625a6cf9e19b99
Add missing condition

Change-Id: I775d34a5f5a91c3b50831b3e3728ffacfe9a0297