mtd: uclinux: do not allow to be built as a module

There isn't any benefit to building the uClinux MTD map as a module as the
rootfs it requires in order to actually be usable is appended to the
kernel image, not the module.  No known system builds it this way either,
so change the option to "bool".

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
CC: Paul Mundt <lethal@linux-sh.org>
CC: Greg Ungerer <gerg@uclinux.org>
CC: uclinux-dev@uclinux.org
CC: linux-mtd@lists.infradead.org
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>