Move grub files from /EFI/BOOT/ to /boot/grub/ on ISO 9660

Move the location of GRUB files so that there is boot-platform
(BIOS vs EFI) neutral place for GRUB-specific files (e.g. configuration
files, background images, etc.).

The EFI binaries are obviosly staying in the default/fallback boot path
`/EFI/BOOT/BOOT*.EFI`, otherwise there will be no UEFI booting.

The /boot/grub/ directory will be used to store BIOS (i386-pc) GRUB
bootsector images, if we get to that.

It is also a requirement for implementing `loopback.cfg` support, since
the file's path must be `/boot/grub/loopback.cfg`.

Implements #206

CHANGELOG.rst

@@ -22,6 +22,8 @@ Changed
 - Honor ``SOURCE_DATE_EPOCH`` in the ``date`` command used by ``profiledef.sh`` of the shipped profiles.
 - Do not duplicate ``grub.cfg`` in both ISO 9660 and the EFI system partition / El Torito image. GRUB will search for
   the ISO volume and load the ``grub.cfg`` from there.
+- Moved GRUB files on ISO 9660 from ``/EFI/BOOT/`` to a boot-platform neutral place ``/boot/grub/``. This does not apply
+  to the EFI binaries that remain in the default/fallback boot path.
 
 Removed
 -------

archiso/mkarchiso

@@ -552,8 +552,8 @@ _make_common_bootmode_grub_copy_to_isofs() {
     if compgen -G "${profile}/grub/!(*.cfg)" &> /dev/null; then
         files_to_copy+=("${profile}/grub/"!(*.cfg))
     fi
-    install -d -m 0755 -- "${isofs_dir}/EFI/BOOT"
-    install -m 0644 -- "${files_to_copy[@]}" "${isofs_dir}/EFI/BOOT/"
+    install -d -m 0755 -- "${isofs_dir}/boot/grub"
+    install -m 0644 -- "${files_to_copy[@]}" "${isofs_dir}/boot/grub/"
 }
 
 # Prepare GRUB configuration files
@@ -608,12 +608,12 @@ fi
 # Load grub.cfg
 if [ "${ARCHISO_HINT}" == 'memdisk' -o -z "${ARCHISO_HINT}" ]; then
     echo 'Could not find the ISO volume!'
-elif [ -e "(${ARCHISO_HINT})/EFI/BOOT/grub.cfg" ]; then
+elif [ -e "(${ARCHISO_HINT})/boot/grub/grub.cfg" ]; then
     export ARCHISO_HINT
     set root="${ARCHISO_HINT}"
-    configfile "(${ARCHISO_HINT})/EFI/BOOT/grub.cfg"
+    configfile "(${ARCHISO_HINT})/boot/grub/grub.cfg"
 else
-    echo "File '(${ARCHISO_HINT})/EFI/BOOT/grub.cfg' not found!"
+    echo "File '(${ARCHISO_HINT})/boot/grub/grub.cfg' not found!"
 fi
 EOF
     grubembedcfg="${grubembedcfg//'%UUID_SEARCH_FILENAME%'/"${uuid_search_filename}"}"