base-files: stage2: add 'tail' to sysupgrade environment

This is used to access footer data in firmare files, and is simpler and
less error-prone than using 'dd' with calculated offsets.
Signed-off-by: NTony Ambardar <itugrok@yahoo.com>

package/base-files/files/lib/upgrade/stage2

@@ -41,7 +41,7 @@ switch_to_ramfs() {
 		pivot_root mount_root reboot sync kill sleep		\
 		md5sum hexdump cat zcat dd tar gzip			\
 		ls basename find cp mv rm mkdir rmdir mknod touch chmod \
-		'[' printf wc grep awk sed cut sort			\
+		'[' printf wc grep awk sed cut sort tail		\
 		mtd partx losetup mkfs.ext4 nandwrite flash_erase	\
 		ubiupdatevol ubiattach ubiblock ubiformat		\
 		ubidetach ubirsvol ubirmvol ubimkvol			\