base-files: board_detect: Address shellcheck warnings

* SC2166 (warning): Prefer [ p ] && [ q ] as [ p -a q ] is not well
   defined.
 * SC2086 (info): Double quote to prevent globbing and word splitting.
 * SC2091 (warning): Remove surrounding $() to avoid executing output
   (or use eval if intentional).

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>
Link: https://github.com/openwrt/openwrt/pull/20831
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

package/base-files/files/bin/board_detect

@@ -4,11 +4,11 @@ CFG=$1
 
 [ -n "$CFG" ] || CFG=/etc/board.json
 
-[ -d "/etc/board.d/" -a ! -s "$CFG" ] && {
+if [ -d "/etc/board.d/" ] && [ ! -s "$CFG" ]; then
 	for a in $(ls /etc/board.d/*); do
-		[ -s $a ] || continue;
-		$(. $a)
+		[ -s "$a" ] || continue
+		(. "$a")
 	done
-}
+fi
 
 [ -s "$CFG" ] || return 1