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

Conflicts:
	tools/releasetools/common.py
Fixed mounting /system when 'override_prop' is enabled.

Change-Id: I59f035498c2a2d65869cb7f7e6ea9e311b9757be