build system: tighten down .xcompile handling some more

Bail out if .xcompile is incomplete or can't be regenerated.

Change-Id: I74adeded7a3e849b25bf65c5b02f67820f29c7e2
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/12477
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

Makefile

@@ -35,6 +35,7 @@
 $(if $(wildcard .xcompile),,$(eval $(shell util/xcompile/xcompile $(XGCCPATH) > .xcompile || rm -f .xcompile)))
 
 .xcompile: util/xcompile/xcompile
+	rm -f $@
 	$< $(XGCCPATH) > $@.tmp
 	\mv -f $@.tmp $@ 2> /dev/null
 
@@ -120,7 +121,13 @@ else
 
 include $(HAVE_DOTCONFIG)
 
-include .xcompile
+-include .xcompile
+
+ifneq ($(XCOMPILE_COMPLETE),1)
+$(shell rm -f .xcompile)
+$(error .xcompile deleted because it's invalid. \
+	Restarting the build should fix that, or explain the problem)
+endif
 
 ifneq ($(CONFIG_MMX),y)
 CFLAGS_x86_32 += -mno-mmx

util/xcompile/xcompile

@@ -355,3 +355,4 @@ for architecture in $SUPPORTED_ARCHITECTURES; do
 		report_arch_toolchain
 	fi
 done
+echo XCOMPILE_COMPLETE:=1