Move MTD-related lines out of the root Makefile. Put them in their
respective directories. Enclose some of these new lines to skip them
when building the SPL. MTD core files and some MTD device drivers are
compiled in a mtd.o object and included in the final object only if
MTD support is required (there are two different symbols for that, one
for U-Boot and one for the SPL).

Now that all defconfigs have been fixed, we can stop the logic where
enabling a command selects the core files to compile. This logic is
broken since selecting a symbol with a 'depends on' will not enforce
this secondary dependency.
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>