tools/boot: Fix universe.sh's ZFS exclusion test

MK_LOADER_ZFS is the current spelling of enabling or disabling ZFS
builds. Use it instead of MK_ZFS.

Sponsored by:		Netflix
Reviewed by:		tsoome
Differential Revision:	https://reviews.freebsd.org/D35896

tools/boot/universe.sh

@@ -94,7 +94,7 @@ for i in \
 	i386/i386 \
 	; do
     ta=${i##*/}
-    dobuild $ta _.boot.${ta}.no_zfs.log "MK_ZFS=no"
+    dobuild $ta _.boot.${ta}.no_zfs.log "MK_LOADER_ZFS=no"
 done
 
 # Build with firewire