treewide: replace `which` with `command -v`

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>