The factory uboot of the Turris Omnia boots with "root=b301", and we
instruct new users to sysupgrade from there (e.g. method 1, step 7).
Currently, this will fail with "Unable to determine upgrade device".
Add a new case to export_bootdevice, which parses the hex argument.
Fixes commit 2e5a0b81 ("mvebu: sysupgrade: sdcard: keep user added ...")
Signed-off-by: Klaus Kudielka <klaus.kudielka@gmail.com>
3a4f587c46