app: aboot: change to pass loaded partition buffers for verification

For "Fastboot boot" command, the loaded dtbo and vbmeta partition buffers
are passed for verification by avb library. This prevent avb to load dtbo
and vbmeta image again.

Change-Id: Ia6533da59e952f4e9bf7b9b26a8a250d38df23a3
Signed-off-by: Saranya Chidura <schidura@codeaurora.org>