Populate recovery DTBO offset correctly

Also, remove recovery_dtbo_offset argument
for mkbootimg as this is calculated based
on sizes of kernel, ramdisk and second
binaries.

Also, modify unpack_bootimg to use the recovery_dtbo_offset
field to extract recovery_dtbo.

Test: make recoveryimage showcommands -j32
Bug: 80207223
unpack_bootimg --boot_img $OUT/recovery.img
diff recovery_dtbo dtbo.img

Change-Id: I588ccc8b739c169b6f78c17ffe554c5562397d98
Merged-In: I588ccc8b739c169b6f78c17ffe554c5562397d98
(cherry picked from commit 26e01bbd)