envsetup: fix 'mmm mka'

Change-Id: Ia5f3fb4813585df21c27ce0035fa3f2b7d6b73e7
Revert "inline kernel dtb build"

This reverts commit 8609a0fb59fa3d943cd33137b0910ced41013102.
This is specific to a device (or vendor) and is confusing people
building for other 8974s. A more generic implementation is required,
and until then device-local mkbootimg overloads can/should be
used.

Change-Id: Ia582f9ca73d454da4ada894b489aea883d6f5186
Make the kernel image format parametric instead of a chain of if/elses

Change-Id: I54bfcdecb8647f7bcf744e72b2de19fcf4e4e7ac