[mb2] Add support for build -j option

Giving -j option to mb2 build will set the build concurrency to
the set value. Otherwise all available CPUs are used for the build.

mb - changed the -j option to set %{_smp_mflags} instead of %{jobs}

Signed-off-by: Juha Kallioinen <juha.kallioinen@jollamobile.com>