serial: Make default_serial_console() a weak function

With this patch it is possible to reimplement default_serial_console()
in board specific code. This will be done in the upcomming PMC440
U-Boot port. This also allows the lwmon board maintainer to
remove the '#if !defined(CONFIG_LWMON) ...' from common/serial.c.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd-electronics.com>