cmd: mbr: Allow 4 MBR partitions without need for extended

Current code allows up to 3 MBR partitions without extended one.
If more than 3 partitions are required, then extended partition(s)
must be used.
This commit allows up to 4 primary MBR partitions without the
need for extended partition.

Add mbr test unit. In order to run the test manually, mmc6.img file
of size 12 MiB or greater is required in the same directory as u-boot.
Test also runs automatically via ./test/py/test.py tool.
Running mbr test is only supported in sandbox mode.

Signed-off-by: Alex Gendin <agendin@matrox.com>
[ And due to some further changes for testing ]
Signed-off-by: Simon Glass <sjg@chromium.org>