Specify -mcpu=cortex-a15 for krait CPU variants.

Bug: 11178216

Change-Id: I9922e4cd5ba27e3857798aae5c84299e26e054ea
build: Handle boot images with custom makefile properly

When a pre-built image should be used, it should be stored in the
target files zip so that it can be used with external signing
processes.

Change-Id: I2661af9ac58af30bb9314b552775046d3abf44e0
Add TWRP MD5 cleaning

Change-Id: I895b8a50acd128df8fa544e0599940ba29c285fc
Merge remote-tracking branch 'ap/kitkat' into kitkat