uboot-envtools: ath79: update ubootenv partion index for gl-ar300m

The block index of u-boot-env changed from mtd1 to mtd3 after upgrading kernel to 5.4.
This patch search the mtd block by label name, work as expect when perform a clean flash.

Signed-off-by: Huangbin Zhan <zhanhb88@gmail.com>

package/boot/uboot-envtools/files/ath79

@@ -19,10 +19,6 @@ buffalo,bhr-4grv2|\
 devolo,magic-2-wifi|\
 engenius,ecb1750|\
 etactica,eg200|\
-glinet,gl-ar300m-lite|\
-glinet,gl-ar300m-nand|\
-glinet,gl-ar300m-nor|\
-glinet,gl-ar300m16|\
 glinet,gl-ar750s-nor|\
 glinet,gl-ar750s-nor-nand|\
 librerouter,librerouter-v1|\
@@ -52,6 +48,14 @@ domywifi,dw33d)
 glinet,gl-ar150)
 	ubootenv_add_uci_config "/dev/mtd1" "0x0" "0x8000" "0x10000"
 	;;
+glinet,gl-ar300m-lite|\
+glinet,gl-ar300m-nand|\
+glinet,gl-ar300m-nor|\
+glinet,gl-ar300m16)
+	idx="$(find_mtd_index u-boot-env)"
+	[ -n "$idx" ] && \
+		ubootenv_add_uci_config "/dev/mtd$idx" "0x0" "0x10000" "0x10000"
+	;;
 netgear,wndr3700|\
 netgear,wndr3700-v2|\
 netgear,wndrmac-v1)