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
BoardConfig.mk.template: Add TARGET_CPU_SMP

This one is easy forget if not in the template

Change-Id: If65e557a661ecc81c6bfc434525b3c9937ff988e