Fix board_detect script to pass shellcheck

Before this change, the board_detect script was
parsing the output of ls to get files in /etc/board.d.
The correct method is to use globs.

Signed-off-by: Matteo Iervasi <matteo.iervasi@edalab.it>

package/base-files/files/bin/board_detect

@@ -5,8 +5,8 @@ CFG=$1
 [ -n "$CFG" ] || CFG=/etc/board.json
 
 [ -d "/etc/board.d/" -a ! -s "$CFG" ] && {
-	for a in $(ls /etc/board.d/*); do
-		[ -s $a ] || continue;
+	for a in /etc/board.d/*; do
+		[ -s "$a" ] || continue;
 		$(. $a)
 	done
 }