grub: install every bootloader only once

Previously, the efi bootloaders were installed in several places.
The main reason was to be compatible with the lipstick grub.cfg
and the ubuntu secure boot grub. This commit unifies the location.
Hence, old lipstick grub.cfg`s are no longer compatible; we need at
least commit
	786f37351916396cb59d21e573d6be372089e516
of the lipstick repository to have a (fully) working grub.cfg.

Makefile

@@ -421,15 +421,10 @@ image_grub_install: $(GRUB_ASSEMBLE_DIR)/grub.x86_64-efi $(GRUB_ASSEMBLE_DIR)/gr
 	$(RSYNC) "/usr/share/grub/themes" "$(IMAGE_DIR)$(GRUB_INSTALL_DIR)/"
 	mkdir -p "$(IMAGE_DIR)$(GRUB_INSTALL_DIR)/fonts"
 	$(RSYNC) "/usr/share/grub/unicode.pf2" "$(IMAGE_DIR)$(GRUB_INSTALL_DIR)/fonts/"
-	#copy efi core files; note that the x64 binary is named grub.efi because grubx64.efi
-	#is occupied by the ubuntu secure boot grub. Furthermore, place a copy in efi/grub
-	#to serve compatibillity with the lipstick grub.cfg
-	#TODO clean up this clutter...
+	#copy efi core files; note that the x64 binary is named grubx64-unsigned.efi because grubx64.efi
+	#is occupied by the ubuntu secure boot grub.
 	mkdir -p "$(IMAGE_DIR)/efi/boot"
-	mkdir -p "$(IMAGE_DIR)/efi/grub"
-	$(RSYNC) --no-p --no-g --no-o "$(GRUB_ASSEMBLE_DIR)/grub.x86_64-efi" "$(IMAGE_DIR)/efi/boot/grub.efi"
-	$(RSYNC) --no-p --no-g --no-o "$(GRUB_ASSEMBLE_DIR)/grub.x86_64-efi" "$(IMAGE_DIR)/efi/grub/grubx64.efi"
-	$(RSYNC) --no-p --no-g --no-o "$(GRUB_ASSEMBLE_DIR)/grub.i386-efi" "$(IMAGE_DIR)/efi/boot/grubia32.efi"
+	$(RSYNC) --no-p --no-g --no-o "$(GRUB_ASSEMBLE_DIR)/grub.x86_64-efi" "$(IMAGE_DIR)/efi/boot/grubx64-unsigned.efi"
 	#our i386 efi bootloader shall be the default:
 	$(RSYNC) --no-p --no-g --no-o "$(GRUB_ASSEMBLE_DIR)/grub.i386-efi" "$(IMAGE_DIR)/efi/boot/bootia32.efi"