ramips-mt7621: add support for UBNT EdgeRouter X

targets/ramips-mt7621

@@ -1,3 +1,10 @@
 # D-Link
 
 device d-link-dir-860l-b1 dir-860l-b1
+
+# Ubiquiti
+
+device ubnt-erx ubnt-erx
+packages '-hostapd-mini'
+factory
+sysupgrade '.tar'