parse the second area of android image

The second area of android image was intended to put a 2nd stage
bootloader but in practice were rarely used (in my knowledge).

An proposal was made to the AOSP to (re)use the second area as the dtb[1],
This patch itself doesn't depend on that proposal being accepted but it won't
be that helpful as well if that proposal won't be accepted. But don't do
any harm as well.

[1] https://android-review.googlesource.com/#/c/417447/


Signed-off-by: Bin Chen <bin.chen@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>