mtd: drop unconditional non LFS build

Buildroot removed non LFS support in the 2015.05 release. We now pass
-D_FILE_OFFSET_BITS=64 unconditionally, so the mtd WITHOUT_LARGEFILE make
variable has no effect.

[Thomas: keep using += for other assigments of MTD_MAKE_OPTS.]

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>