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 remote-tracking branch 'cm/cm-11.0' into cm-11.0
Merge pull request #3 from androidarmv6/cm-11.0

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