dual-image: Detect the running image slot

Add detect-slot-aspeed to detect which slot it is running on, and
assign the member variable, which will be used in future commits.

Tested:
* In normal

dual-image: Detect the running image slot

Add detect-slot-aspeed to detect which slot it is running on, and
assign the member variable, which will be used in future commits.

Tested:
* In normal case, verify the primary slot is 0 and secondary slot
is 1.
* Manually assign value 1 in /run/media/slot, and verify the slot
number is reversed.
* Manually trigger wdt2 timeout and verify in secondary flash that
the slot number is correct, that the running slot is 1 and the
other slot is 0.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: I69edb1961b4c4bb79e7897c38b9ce8dded42f829

show more ...