Mk: allow ROOT fallback, but make sure it's there

Mk/tree.mk

 all:
 
-ROOT?=	/usr/local
-
 .for TARGET in ${TREES} ${EXTRAS}
 
 .if "${TREES_${TARGET}}" == ""
@@ -9,6 +7,9 @@ TREES_${TARGET}=${TARGET}
 .endif
 
 .if "${ROOT_${TARGET}}" == ""
+.if "${ROOT}" == ""
+.error "No ROOT directory set for target: ${TARGET}"
+.endif
 ROOT_${TARGET}=${ROOT}
 .endif
 

src/Makefile

 TREES=		boot etc opnsense sbin wizard www
+ROOT=		/usr/local
 ROOT_boot=	/
 
 EXTRAS=		bootstrap