mxs: start a console on USB gadget serial ports

The I2SE Duckbill devices are pen-drive like and thus the usage
of the USB gadget subsystem is highly indicated.

Spawning a console if a serial gadget is configured saves
us from connecting a physical debug uart.

On Olimex OLinuXino Maxi/Mini boards, this does not hurt.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

target/linux/mxs/base-files/etc/inittab

@@ -1,3 +1,4 @@
 ::sysinit:/etc/init.d/rcS S boot
 ::shutdown:/etc/init.d/rcS K shutdown
 ::askconsole:/usr/libexec/login.sh
+ttyGS0::askfirst:/usr/libexec/login.sh