bootstd: Add test for bootmeth_android

Add a unit test for testing the Android bootmethod.

This requires another mmc image (mmc7) to contain the following partitions:
- misc: contains the Bootloader Control Block (BCB)
- boot_a: contains a fake generic kernel image
- vendor_boot_a: contains a fake vendor_boot image

Also add BOOTMETH_ANDROID as a dependency on sandbox so that we can test
this with:

$ ./test/py/test.py --bd sandbox --build -k test_ut # to build the mmc7.img
$ ./test/py/test.py --bd sandbox --build -k bootflow_android

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>