mxs: switch to askconsole

Remove the dedicated tty name and use askconsole to spawn the console
on serial debug uart.

Signed-off-by: Michael Heimpold <[email protected]>

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

@@ -1,3 +1,3 @@
 ::sysinit:/etc/init.d/rcS S boot
 ::shutdown:/etc/init.d/rcS K shutdown
-ttyAMA0::askfirst:/usr/libexec/login.sh
+::askconsole:/usr/libexec/login.sh