build: Handle custom boot images 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.

Original-Change-Id: I2661af9ac58af30bb9314b552775046d3abf44e0
Change-Id: I10b3bd0bb33489b8ffb26d16d002f8dd6ff405ad
build: Use project pathmap for recovery

Change-Id: I6339ac77b899a43db21261d587252b65cb58ad79