cmd: nand/sf: isolate legacy code

The 'sf' command is not supposed to rely on the MTD stack, but both
'sf' and 'nand' commands use helpers located in mtd_uboot.c. Despite
their location, these functions do not depend at all on the MTD
stack.

This file (drivers/mtd/mtd_uboot.c) is only compiled if CONFIG_MTD is
selected, which is inconsistent with the current situation. Solve this
by moving these three functions (which are only used by the above two
commands) out of mtd_uboot.c and put them in a C file only compiled
with cmd/sf.c and cmd/nand.c.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
[trini: Don't export get_part function now]
Signed-off-by: Tom Rini <trini@konsulko.com>