treewide: replace which with command -v

Fix shellcheck SC2230
> which is non-standard. Use builtin 'command -v' instead.

Once applied to everything concerning OpenWrt we can disable the busybox
feature `which` and save 3.8kB.

Signed-off-by: Paul Spooren <mail@aparcar.org>

net/ddns-scripts/files/ddns.defaults

@@ -33,7 +33,7 @@ timer2dhms() {
 
 # using function to not confuse function calls with existing ones inside /lib/functions.sh
 update_config() {
-	uc_uci="$(which uci) -q"	# ignore errors
+	uc_uci="$(command -v uci) -q"	# ignore errors
 	uc_cfg=""
 	uc_name=""
 	uc_var=""