ath79: expand factory.bin support for some Senao Engenius boards

Newer EnGenius software that still uses the tar.gz platform
  instead of the custom header requires more checks for upgrading,
  but their script includes a way to skip them...
  the existence of a file in the tar.gz called failsafe.bin

  Their upgrade script has these lines:

  \#pass check when upload with full image file
  [ "${errcode}" -eq "1" ] && [ -f failsafe.bin ] && errcode="0"

  This overrides the script's "errcode" variable
  which can be set if any of the following actions/checks fail:

  - untarring of the upload

  - magic number for kernel: "2705"

  - magic num for rootfs: "7371" or "6873"

  - md5sums for each file in the format
	filename:md5

  - existence of a file matching FWINFO*
	that it has boardname in the name somewhere (grep)
	that the 4th field of separator "-" is at least 3 (version)

  Otherwise we would need to generate md5sums in this strange format
  and touch a file with specific requirements in the name.

  This does not effect boards where the advanced checks do not apply.

Signed-off-by: Michael Pratt <mcpratt@pm.me>
[fixed SoB to match From:]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/ath79/image/generic.mk

@@ -74,6 +74,7 @@ endef
 define Build/engenius-tar-gz
 	-[ -f "$@" ] && \
 	mkdir -p $@.tmp && \
+	touch $@.tmp/failsafe.bin && \
 	echo '#!/bin/sh' > $@.tmp/before-upgrade.sh && \
 	echo ': > /tmp/_sys/sysupgrade.tgz' >> $@.tmp/before-upgrade.sh && \
 	$(CP) $(KDIR)/loader-$(DEVICE_NAME).uImage \