boost: add an option to run cmds in parallel

Add the same -jN parameter as used for make.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/boost/boost.mk

@@ -70,7 +70,7 @@ define BOOST_CONFIGURE_CMDS
 endef
 
 define BOOST_INSTALL_TARGET_CMDS
-	(cd $(@D) && ./b2 -q -d+2 \
+	(cd $(@D) && ./b2 -j$(BR2_JLEVEL) -q -d+2 \
 	--user-config=$(@D)/user-config.jam \
 	$(BOOST_OPT) \
 	--prefix=$(TARGET_DIR)/usr \
@@ -78,7 +78,7 @@ define BOOST_INSTALL_TARGET_CMDS
 endef
 
 define BOOST_INSTALL_STAGING_CMDS
-	(cd $(@D) && ./bjam -d+2 \
+	(cd $(@D) && ./bjam -j$(BR2_JLEVEL) -d+2 \
 	--user-config=$(@D)/user-config.jam \
 	$(BOOST_OPT) \
 	--prefix=$(STAGING_DIR)/usr \