app: aboot: Add support for new boot image format
app: aboot: Add support for new boot image format

From Android N, unused fields in boot image header are used for
os version and patch level information respectively. One of the unused
fields were previously used for differentiating between appended dtb img
and dt.img. Make changes so that new boot image generated for N
can boot properly.

CRs-Fixed: 1041666
Change-Id: I5073e285890f3d524eeb512a3a44e4a4742d817b

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