zephyr: Check zephyr,uart-mcumgr as candidate for serial recovery

The commit modifies selection of boot serial UART by first checking
the Zephyr chosen zephyr,uart-mcumgr and then reverting to the
zephyr,console, as a secondary candidate.
In case when both nodes are present and point to the same device,
error will be reported.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>