configs/beaglebone_defconfig: use default console device

OMAP kernels use 8250 driver by default. Hence the name of
the console device is not /dev/ttyO0 but /dev/ttyS0.
Use /dev/console in order to handle the console independently
of the selected driver.

Tested in BeagleBone Black board.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>