Fix encrypted MBR install

The pools are exported and reimported in order to write the bootcode
This causes an error when the bootpool is later mounted by common code
The bootpool is now imported with the -N flag to prevent mounting

Reported by:	Michael Dexter
MFC after:	5 days

usr.sbin/bsdinstall/scripts/zfsboot

@@ -1384,8 +1384,9 @@ zfs_create_boot()
 			     "-o altroot=\"$BSDINSTALL_CHROOT\"" "$zroot_name" ||
 			     return $FAILURE
 		if [ "$ZFSBOOT_BOOT_POOL" ]; then
+			# Import the bootpool, but do not mount it yet
 			f_eval_catch $funcname zpool "$ZPOOL_IMPORT_WITH_OPTIONS" \
-				     "-o altroot=\"$BSDINSTALL_CHROOT\"" \
+				     "-o altroot=\"$BSDINSTALL_CHROOT\" -N" \
 				     "$bootpool_name" || return $FAILURE
 		fi
 	fi