Revert "build: Handle custom boot images properly"

We don't want to force this since we need to be able to
re-sign boot images with proper keys, which requires rebuilding
of the images.

This also has an error in it from merging (dropped _MK).

This reverts commit 22913dea.

Change-Id: Id1353e9883c3f24bbc10e685f43bede779430025