build: switch default target from ar71xx to ath79

ar71xx is in the process of being deprecated as a target accepting new
devices.  The replacement target for the same hardware is DTS
based ath79.

Switch the default build target selection from ar71xx to ath79.

This is intended to encourage DTS takeup & support for ath79 and longer
term will also aid kernel upstream support.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

scripts/target-metadata.pl

@@ -170,7 +170,7 @@ EOF
 	print <<EOF;
 choice
 	prompt "Target System"
-	default TARGET_ar71xx
+	default TARGET_ath79
 	reset if !DEVEL
 	
 EOF