app: aboot: Retrieve dt size while booting from flash

Dt size should get from boot image header while booting
from flash using either signed or unsigned kernel,
if OSVERSION_IN_BOOTIMAGE flag not defined.

CRs-Fixed: 1066696
Change-Id: Ic4bbcbd83c8077df4617f1e6c3db3d29203f78aa
1 file changed