image:grub: set prefix to /grub to avoid conflicts

with the ubuntu secure boot grub located in /boot/grub.

Makefile

@@ -395,7 +395,7 @@ $(GRUB_ASSEMBLE_DIR)/grub.x86_64-efi $(GRUB_ASSEMBLE_DIR)/grub.i386-efi : $(GRUB
 image_grub_mkimage_mbr: $(GRUB_ASSEMBLE_DIR)/grub.i386-pc
 $(GRUB_ASSEMBLE_DIR)/grub.i386-pc : | $(WORKSPACE)
 	mkdir -p "$(GRUB_ASSEMBLE_DIR)"
-	grub-mkimage --prefix "(hd0,msdos1)/boot/grub" \
+	grub-mkimage --prefix "(hd0,msdos1)/grub" \
                 --output "$@" --format "i386-pc" \
                 $(IMAGE_GRUB_MBR_MODULES)
 

contrib/image/grub_early.cfg

 if [ -z "$prefix" -o ! -e "$prefix" ]; then
 	if ! search --file --set=root /.disk/info; then
 		search --file --set=root /.disk/mini-info
-	set prefix=($root)/boot/grub
+	set prefix=($root)/grub
 if [ -e $prefix/x86_64-efi/grub.cfg ]; then
 	source $prefix/x86_64-efi/grub.cfg
 else