Unifi AC: remove bs-partition ro-flag

This patch allows to modify the bs-partition on the AC Mesh.

Without the patch, the bs partition on the AC Mesh is locked
forever. This can be a problem if
a) the bs partition has not been updated correctly during initial flash
b) the sysupgrade mechanism is altered by OpenWRT in order to account for
the flip flop layout

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>

build_patches/openwrt/0031-ar71xx-remove-bs-partition-ro-flag-for-UniFi-AC.patch

+From e873363e0541d8ded05635e8341efe564c5d9cff Mon Sep 17 00:00:00 2001
+From: Adrian Schmutzler <freifunk@adrianschmutzler.de>
+Date: Tue, 10 Apr 2018 19:04:13 +0200
+Subject: [PATCH] ar71xx: remove bs-partition ro-flag for UniFi AC
+
+This removes the read-only flag from the bs (bootselect) partition
+on UniFi AC devices. This allows to correct the indicator from which
+partition the device is booting its kernel from.
+
+See also:
+ - freifunk-gluon/gluon#1301freifunk-gluon/gluon#1301, ar71xx:  UBNT UniFi AP-AC Mesh/Lite/Pro sysupgrade broken
+ - https://bugs.lede-project.org/index.php?do=details&task_id=662
+
+Signed-off-by: David Bauer <mail@david-bauer.net>
+[Backported to our lede-17.01 snapshot]
+Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
+---
+ target/linux/ar71xx/image/ubnt.mk | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/target/linux/ar71xx/image/ubnt.mk b/target/linux/ar71xx/image/ubnt.mk
+index 68fe8ad..65ed708 100644
+--- a/target/linux/ar71xx/image/ubnt.mk
++++ b/target/linux/ar71xx/image/ubnt.mk
+@@ -82,7 +82,7 @@ define Device/ubnt-unifiac
+   DEVICE_PACKAGES := kmod-usb-core kmod-usb-ohci kmod-usb2
+   DEVICE_PROFILE := UBNT
+   IMAGE_SIZE := 7744k
+-  MTDPARTS = spi0.0:384k(u-boot)ro,64k(u-boot-env)ro,7744k(firmware),7744k(ubnt-airos)ro,128k(bs)ro,256k(cfg)ro,64k(EEPROM)ro
++  MTDPARTS = spi0.0:384k(u-boot)ro,64k(u-boot-env)ro,7744k(firmware),7744k(ubnt-airos)ro,128k(bs),256k(cfg)ro,64k(EEPROM)ro
+   IMAGES := sysupgrade.bin
+   IMAGE/sysupgrade.bin = append-kernel | pad-to $$$$(BLOCKSIZE) | append-rootfs | pad-rootfs | check-size $$$$(IMAGE_SIZE)
+ endef
+-- 
+2.7.4
+