At least with grub 2.02~beta3-1 the prefix is mandatory:
| Usage: grub-mkimage [OPTION...] [OPTION]... [MODULES]
| Try 'grub-mkimage --help' or 'grub-mkimage --usage' for more information.
| Prefix not specified (use the -p option).
Set it to what's the default e.g. in grub 2.02~beta2-36
as documented in its manpage:
| -p, --prefix=DIR
| set prefix directory [default=/boot/grub]
Related grub git commit message:
| commit
1440b7ebae47e1cb710e920bec9d88c4d75ff432
| Author: Andrey Borzenkov <arvidjaar@gmail.com>
| Date: Sat Jan 25 19:54:51 2014 +0400
|
| do not set default prefix in grub-mkimage
|
| Default prefix is likely wrong on Unix and completely wrong on Windows.
| Let caller set it explicitly to avoid any ambiguity.
Thanks: Darshaka Pathirana