ar71xx: Added missing support for Linksys E2100L

It's based on the WRT160NL according to
https://wiki.openwrt.org/toh/linksys/e2100l

Based on research done here: https://forum.openwrt.org/viewtopic.php?id=24244
and here: https://forum.openwrt.org/viewtopic.php?pid=120791#p120791 this patch
was conceived.

Signed-off-by: Gad Krumholz <gad.krumholz@gmail.com>
[Jo-Philipp Wich: fix ordering, move addpattern change into separate commit]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

target/linux/ar71xx/base-files/lib/ar71xx.sh

@@ -577,6 +577,9 @@ ar71xx_board_detect() {
 	*"DW33D")
 		name="dw33d"
 		;;
+	*E2100L)
+		name="e2100l"
+		;;
 	*"EAP120")
 		name="eap120"
 		tplink_pharos_board_detect

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -495,6 +495,7 @@ platform_check_image() {
 
 		return 0
 		;;
+	e2100l|\
 	mynet-rext|\
 	wrt160nl)
 		cybertan_check_image "$1" && return 0