Edify: Add/Update abort messages for some asserts

* If a bootloader or baseband assert is not satisfied, print the
  versions that are supported by the package as well as the version
  on the device.

Change-Id: I958d49281c51bd4e60d596a727bb94cfc4a21909
Allow specifying kernel toolchain and prefix

Change-Id: Ib75cfe772e073f2196455f3e3dfba247f6b3feff
build: Only mount /system if not mounted

The commit on which this depends takes care of the mount error if /data
is already mounted during install. This commit takes care of the error
if /system is already mounted.

Change-Id: I97d0540676a7ce98762545e76ecb1a7f6335d204
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: add color to chromium prebuilt check message

Change-Id: I1063dc744f7f6ab618e10c9e3b5132af1cbab490