boost: add target build step

The build step for the target was missing, causing the compilation to happen
during the staging install step.  Add a dedicated build step like we do for
the host build.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/boost/boost.mk

@@ -128,6 +128,14 @@ define BOOST_CONFIGURE_CMDS
 	echo "" >> $(@D)/user-config.jam
 endef
 
+define BOOST_BUILD_CMDS
+	(cd $(@D) && ./bjam -j$(PARALLEL_JOBS) -q \
+	--user-config=$(@D)/user-config.jam \
+	$(BOOST_OPTS) \
+	--ignore-site-config \
+	--layout=$(BOOST_LAYOUT))
+endef
+
 define BOOST_INSTALL_TARGET_CMDS
 	(cd $(@D) && ./b2 -j$(PARALLEL_JOBS) -q \
 	--user-config=$(@D)/user-config.jam \