waf: use make -j on build of ChibiOS core

using number of jobs from waf

Tools/ardupilotwaf/chibios.py

@@ -354,7 +354,7 @@ def build(bld):
         common_src += [bld.bldnode.find_or_declare('ap_romfs_embedded.h')]
     ch_task = bld(
         # build libch.a from ChibiOS sources and hwdef.h
-        rule="BUILDDIR='${BUILDDIR_REL}' CHIBIOS='${CH_ROOT_REL}' AP_HAL=${AP_HAL_REL} ${CHIBIOS_BUILD_FLAGS} ${CHIBIOS_BOARD_NAME} '${MAKE}' lib -f '${BOARD_MK}'",
+        rule="BUILDDIR='${BUILDDIR_REL}' CHIBIOS='${CH_ROOT_REL}' AP_HAL=${AP_HAL_REL} ${CHIBIOS_BUILD_FLAGS} ${CHIBIOS_BOARD_NAME} '${MAKE}' -j%u lib -f '${BOARD_MK}'" % bld.options.jobs,
         group='dynamic_sources',
         source=common_src,
         target=bld.bldnode.find_or_declare('modules/ChibiOS/libch.a')