boards: thingy53: start network core before 802.15.4 driver

The nRF5 802.15.4 driver tries to establish a rpmsg channel
with the network core, so the network core must be started
earlier. Therefore, decrease the priority level of the task
responsible for initializing the board.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>