misc: fix the MAKE_PARALLEL_JOBS for 'make verify' target

MAKE_PARALLEL_JOBS was prepending "-j", which in case of nested targets like "verify"
resulted in ninja being executed with "-j -j" flags.

Solution: prepend -j only at the moment of setting the MAKE_PARALLEL_FLAGS variable,
this keeps MAKE_PARALLEL_JOBS intact regardless of depth of the job call tree.

Type: make
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
Change-Id: I10a69fe527758e5a862852a9f2102aeaca30e88f