This new define enables mtdcore.c compilation and with this we can
select the MTD device infrastructure needed for the reworked mtdparts
command.

We now have the 2 MTD infrastructure defines, CONFIG_MTD_DEVICE and
CONFIG_MTD_PARTITIONS. CONFIG_MTD_DEVICE is needed (as explained above)
for the "mtdparts" command and CONFIG_MTD_PARTITIONS is needed for UBI.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Scott Wood <scottwood@freescale.com>