bcm53xx: upgrade: fix RAMFS_COPY_*

Fixes: 30f61a34b4 "base-files: always use staged sysupgrade"
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

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

@@ -48,7 +48,7 @@ switch_to_ramfs() {
 		/bin/sleep /bin/zcat /usr/bin/bzcat /usr/bin/printf /usr/bin/wc \
 		/bin/cut /usr/bin/printf /bin/sync /bin/mkdir /bin/rmdir	\
 		/bin/rm /usr/bin/basename /bin/kill /bin/chmod /usr/bin/find \
-		/bin/mknod /bin/touch
+		/bin/mknod /bin/touch /bin/sed
 
 	install_bin /sbin/mtd
 	install_bin /sbin/mount_root