GRMLBASE/44-grub: explicitely set grub-mkimage's prefix path

GRMLBASE/44-grub: explicitely set grub-mkimage's prefix path

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