mtd: fix build issues on older hosts

Building the MTD test programs requires the MS_DIRSYNC, which is not
necessarily available on old build machines. But obviously, MTD test
programs are not needed, so we can simply disable them, as they were
prior to the migration to mtd 2.0.

Fixes:

  http://autobuild.buildroot.net/results/21e1ad2a4560b6d3ba6490d20ae064246e66d5c1/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>