shim: don't let build modify shim.config

Currently building the shim will modify shim.config in case some config
option was added or modified in the hypervisor.

Avoid that by copying shim.config to an intermediate file instead.

(cherry picked from commit 8717e74)
Cherry-pick-repository: https://xenbits.xen.org/gitweb/?p=xen.git
Issue: ALB-3594
Signed-off-by: Juergen Gross <jgross@suse.com>

tools/firmware/xen-dir/Makefile

@@ -41,16 +41,12 @@ linkfarm.stamp: $(DEP_DIRS) $(DEP_FILES) FORCE
 $(D): linkfarm.stamp
 	$(MAKE) -C $(D)/xen distclean
 
-.PHONY: shim-%config
-shim-%config: $(D) FORCE
-	$(MAKE) -C $(D)/xen $*config \
-		XEN_CONFIG_EXPERT=y \
-		KCONFIG_CONFIG=$(CURDIR)/shim.config
-
-xen-shim: $(D) shim-olddefconfig
-	$(MAKE) -C $(D)/xen build \
-		XEN_CONFIG_EXPERT=y \
-		KCONFIG_CONFIG=$(CURDIR)/shim.config
+$(D)/xen/.config: shim.config $(D)
+	cp $< $@
+	$(MAKE) -C $(@D) olddefconfig XEN_CONFIG_EXPERT=y
+
+xen-shim: $(D)/xen/.config
+	$(MAKE) -C $(<D) build XEN_CONFIG_EXPERT=y
 	ln -sf $(D)/xen/xen $@
 	ln -sf $(D)/xen/xen-syms $@-syms