util/nvmutil: use x, not ?, for random characters

A user reported that '?' causes an error on zsh. See:

https://codeberg.org/libreboot/lbmk/issues/261

For example:

./mk inject libreboot-XXXXXX.tar.xz setmac ??:??:??:??:??:??

The user got:

 zsh: no matches found: ??:??:??:??:??:??

The mitigation here is to double-quote, e.g.:

./mk inject libreboot-XXXXXX.tar.xz setmac "??:??:??:??:??:??"

However, a lot of people won't do that. Therefore, I will
retain the current behaviour but support x/X for randomness.

Now lbmk uses x by default, instead. I will now update the
documentation, accordingly.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/inject.sh

@@ -328,7 +328,7 @@ inject()
 	eval "`setvars "" nukemode new_mac xchanged`"
 
 	archive="$1";
-	new_mac="??:??:??:??:??:??"
+	new_mac="xx:xx:xx:xx:xx:xx"
 
 	[ $# -gt 1 ] && case "$2" in
 	nuke)