ramips: Fix sysupgrade for Xiaomi mir3g

This mostly reverts the original commit e9929ebeea ("ramips: Fix
sysupgrade for Xiaomi mir3g") and replaces it with setting the
BOARD_NAME to the old value.

This way the folder in the tar will be named sysupgrade-mir3g and not
sysupgrade-xiaomi_mir3g and the sysupgrade in OpenWrt 18.06 can find it.

Without this change sysupgrade from 18.06 to 19.07 is only possible with
the -F option.

I tested the following sysupgrades successfully without -F
18.06 -> 19.07
19.07 -> master
master -> 19.07

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/ramips/image/mt7621.mk

@@ -269,25 +269,26 @@ define Device/xiaomi_mir3p
 endef
 TARGET_DEVICES += xiaomi_mir3p
 
-define Device/mir3g
+define Device/xiaomi_mir3g
   DTS := MIR3G
   BLOCKSIZE := 128k
   PAGESIZE := 2048
   KERNEL_SIZE := 4096k
   IMAGE_SIZE := 32768k
   UBINIZE_OPTS := -E 5
+  BOARD_NAME := mir3g
   IMAGES += kernel1.bin rootfs0.bin
   IMAGE/kernel1.bin := append-kernel
   IMAGE/rootfs0.bin := append-ubi | check-size $$$$(IMAGE_SIZE)
   IMAGE/sysupgrade.bin := sysupgrade-tar | append-metadata
   DEVICE_TITLE := Xiaomi Mi Router 3G
   SUPPORTED_DEVICES += R3G
-  SUPPORTED_DEVICES += xiaomi,mir3g
+  SUPPORTED_DEVICES += mir3g
   DEVICE_PACKAGES := \
 	kmod-mt7603 kmod-mt76x2 kmod-usb3 kmod-usb-ledtrig-usbport wpad-basic \
 	uboot-envtools
 endef
-TARGET_DEVICES += mir3g
+TARGET_DEVICES += xiaomi_mir3g
 
 define Device/mt7621
   DTS := MT7621