EFI_MOUNTPOINT is conventionally /boot/efi or /boot/EFI or something
like that, and (on my machine) has directories within that called
{Boot, nixos, gummiboot}.

This patch does two things:

1) Xen apparently wants to put files in
$(EFI_MOUNTPOINT)/efi/$(EFI_VENDOR) - we remove the duplicate 'efi' name
because I can't see why we have it

2) Ensures the said directory exists


--- a/xen/Makefile	2016-01-08 01:50:58.028045657 +0000
+++ b/xen/Makefile	2016-01-08 01:51:33.560268718 +0000
@@ -49,7 +49,9 @@
 		ln -sf $(T)-$(XEN_FULLVERSION).efi $(D)$(EFI_DIR)/$(T)-$(XEN_VERSION).efi; \
 		ln -sf $(T)-$(XEN_FULLVERSION).efi $(D)$(EFI_DIR)/$(T).efi; \
 		if [ -n '$(EFI_MOUNTPOINT)' -a -n '$(EFI_VENDOR)' ]; then \
-			$(INSTALL_DATA) $(TARGET).efi $(D)$(EFI_MOUNTPOINT)/efi/$(EFI_VENDOR)/$(T)-$(XEN_FULLVERSION).efi; \
+			[ -d $(D)$(EFI_MOUNTPOINT)/$(EFI_VENDOR) ] || \
+			  $(INSTALL_DIR) $(D)$(EFI_MOUNTPOINT)/$(EFI_VENDOR) ;\
+			$(INSTALL_DATA) $(TARGET).efi $(D)$(EFI_MOUNTPOINT)/$(EFI_VENDOR)/$(T)-$(XEN_FULLVERSION).efi; \
 		elif [ "$(D)" = "$(patsubst $(shell cd $(XEN_ROOT) && pwd)/%,%,$(D))" ]; then \
 			echo 'EFI installation only partially done (EFI_VENDOR not set)' >&2; \
 		fi; \
@@ -69,7 +69,7 @@
 	rm -f $(D)$(EFI_DIR)/$(T)-$(XEN_VERSION).$(XEN_SUBVERSION).efi
 	rm -f $(D)$(EFI_DIR)/$(T)-$(XEN_VERSION).efi
 	rm -f $(D)$(EFI_DIR)/$(T).efi
-	rm -f $(D)$(EFI_MOUNTPOINT)/efi/$(EFI_VENDOR)/$(T)-$(XEN_FULLVERSION).efi
+	rm -f $(D)$(EFI_MOUNTPOINT)/$(EFI_VENDOR)/$(T)-$(XEN_FULLVERSION).efi
 
 .PHONY: _debug
 _debug: