bootloader: Add an aboot (Android) bootloader backend

bootloader: Add an aboot (Android) bootloader backend

aboot is special in that it packages kernel, initrd, cmdline, dtb and
signature one combined image (similar to upcoming unified kernel
images). This is then loaded as an image into an aboot partition.

This image is signed by the OS vendor and covers everything in the
image. So locally on the deployed system it should not be possible to
boot an unsigned image (unless signature checking is turned off).

We call a shell script aboot-deploy when it is required to write a new
image to the aboot partition (a file typically starting with aboot and
ending in .img extension). This shell script may also read some
configurations from a .cfg file.

Signed-off-by: Eric Curtin <ecurtin@redhat.com>