base-files: replace backticks by $(...)

This replaces deprecated backticks by more versatile $(...) syntax.

This does not touch lib/upgrade/nand.sh, as there replacement is
not trivial.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

package/base-files/files/bin/board_detect

@@ -5,7 +5,7 @@ CFG=$1
 [ -n "$CFG" ] || CFG=/etc/board.json
 
 [ -d "/etc/board.d/" -a ! -s "$CFG" ] && {
-	for a in `ls /etc/board.d/*`; do
+	for a in $(ls /etc/board.d/*); do
 		[ -x $a ] || continue;
 		$(. $a)
 	done