revert change, variable is used in update-patches

mk/buildhlp.mk

@@ -18,7 +18,11 @@ WRKBUILD?=		${WRKSRC}
 # This is where make install (fake) writes to
 WRKINST?=		${WRKDIR}/fake-${CPU_ARCH}/root
 
-_CHECKSUM_COOKIE=	${WRKDIR}/.checksum_done
+ifeq ($(strip ${NO_CHECKSUM}),)
+_CHECKSUM_COOKIE=      ${WRKDIR}/.checksum_done
+else
+_CHECKSUM_COOKIE=
+endif
 
 post-extract: