app: aboot: add target boot params

Make aboot to get target specific boot params so that boot params can be
appended to kernel command line.
Also fix mdmkrypton to provide 'root=' as target boot param to get rid of
hardcoded root device name from boot image.

Change-Id: I10129a0249438b8bc7da1ea8a8c0097489e6e289
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
3 files changed