mtd: Introduce CONFIG_MTD_DEVICE to select compilation of mtdcore.o

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>