Bluetooth: Host: Fix monitor UART selection

Most boards set zephyr,bt-mon-uart to point at the same device as
zephyr,console. It's still useful to have the possibility of having these
as two different devices, however it's useful to fall back to the UART
console in case a dedicated monitor UART hasn't been specified.

This also ensures that the fallback only happens if the console UART isn't
enabled, but a DT chosen property exists for it. Additionally, we now get a
user friendly error in case the Bluetooth UART monitor feature has been
enabled in Kconfig but there isn't a suitable UART available for it in
devicetree.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>