package/most: disable parallel build

package/most: disable parallel build

The hand-written Makefile is not parallel-build safe: the objs directory
is created in a separate rule, but the rules that create files in that
directory don't depend on it.

Although this relatively simple to fix, upstream clearly doesn't care
about parallel builds so it is likely to break again in the future.
Since most consists of just a dozen source files, just disable parallel
build.

Fixes:
http://autobuild.buildroot.net/results/c7540203a8eadad3f324f0d7e7fe66a526d4a2e9

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Sven Oliver Moll <buildroot@svol.li>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>