Use bootstrapped install(1) install of tools/install.sh in world stage

This should be noticeably faster due to fewer processes being forked and
also handles other flags such as -S or writing to METALOG.

Reviewed By:	brooks
Differential Revision: https://reviews.freebsd.org/D26039

Makefile.inc1

@@ -765,7 +765,7 @@ KTMAKE=		\
 
 # world stage
 WMAKEENV=	${CROSSENV} \
-		INSTALL="sh ${.CURDIR}/tools/install.sh" \
+		INSTALL="${INSTALL_CMD} -U" \
 		PATH=${TMPPATH} \
 		SYSROOT=${WORLDTMP}