The --grub-mkimage option of grub-install

[Tom Horsley]

The grub that appears on my fedora 28 system doesn't seem
to have much relation to the documentation :-(.

I'm trying to install grub on a USB key and embed
a config to search for the UUID of the USB key root
partition.

The docs say I can pass my own copy of grub-mkimage
to grub-install using the --grub-mkimage option.

It takes the option, but it never invokes my custom
grub-mkimage (which just execs the real grub-mkimage
passing an extra -c config option to do my search
directives). I know it isn't called because I put
in a command to echo to a log file which never happens.

So how do I embed a config in a grub install for
real using the latest undocumented version of grub?