treewide: replace which with command -v

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

Using `command -v` is POSIX compliant while `which` is not.  Also to
mention, `command -v` is a shell builtin whereas `which` is a separate
busybox applet.

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

Acked-by: Stijn Tintel <stijn@linux-ipv6.be>
Signed-off-by: Paul Spooren <mail@aparcar.org>
[also replace cases in zram-swap]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

package/base-files/files/lib/upgrade/stage2

@@ -45,7 +45,7 @@ switch_to_ramfs() {
 		snapshot snapshot_tool					\
 		$RAMFS_COPY_BIN
 	do
-		local file="$(which "$binary" 2>/dev/null)"
+		local file="$(command -v "$binary" 2>/dev/null)"
 		[ -n "$file" ] && install_bin "$file"
 	done
 	install_file /etc/resolv.conf /lib/*.sh /lib/functions/*.sh /lib/upgrade/*.sh /lib/upgrade/do_stage2 /usr/share/libubox/jshn.sh $RAMFS_COPY_DATA