stubdom: fix build with disabled pv-grub

Today the build will fail if --disable-pv-grub as a parameter of
configure, as the main Makefile will unconditionally try to build a
32-bit pv-grub stubdom.

Fix that by introducing a pv-grub-if-enabled target in
stubdom/Makefile taking care of this situation.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Ian Jackson <iwj@xenproject.org>

Makefile

@@ -72,7 +72,7 @@ build-tools-oxenstored: build-tools-public-headers
 build-stubdom: mini-os-dir build-tools-public-headers
 	$(MAKE) -C stubdom build
 ifeq (x86_64,$(XEN_TARGET_ARCH))
-	XEN_TARGET_ARCH=x86_32 $(MAKE) -C stubdom pv-grub
+	XEN_TARGET_ARCH=x86_32 $(MAKE) -C stubdom pv-grub-if-enabled
 endif
 
 .PHONY: build-docs
@@ -143,7 +143,7 @@ install-tools: install-tools-public-headers
 install-stubdom: mini-os-dir install-tools
 	$(MAKE) -C stubdom install
 ifeq (x86_64,$(XEN_TARGET_ARCH))
-	XEN_TARGET_ARCH=x86_32 $(MAKE) -C stubdom install-grub
+	XEN_TARGET_ARCH=x86_32 $(MAKE) -C stubdom install-grub-if-enabled
 endif
 
 .PHONY: tools/firmware/seabios-dir-force-update