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
Merge pull request #29 from CyanogenMod/cm-11.0

build: Handle boot images with custom makefile properly