Makefile.inc: Ensure update of build.h

There were so many pitfalls that I wrote my own version of this even-
tually. This version is inspired by the procedure of Alex Thiessen[1].
Instead of generating a `build.h` on demand, we always generate a tem-
porary version that, if it differs from the current one, is added as
a dependency.

As we use .SECONDEXPANSION on the prerequisites, special care is taken
that we won't generate the file twice. As it would be too late to add
the dependency if we'd run `genbuild_h.sh` inside a recipe, we have
to run it through the `$(shell)` function. But that brings us to the
next issue: The make variables used by `genbuild_h.sh` are not expor-
ted to this shell like they would be in a recipe. So we export them
manually. We could also make these variables explicit parameters of
`genbuild_h.sh` instead.

An alternative to always creating the temporary `build.h` would be
to add a phony target as dependency instead, and finally calling
`genbuild_h.sh` again in case we need an update. But, um, we create
so many files anyway...

[1] https://review.coreboot.org/25685

Change-Id: I311cf610eabae873c70f2985fc7a09acec8061f0
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/28197
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

Makefile.inc

@@ -464,11 +464,24 @@ export $(COREBOOT_EXPORTS)
 
 #######################################################################
 # generate build support files
-$(obj)/build.h: .xcompile
+
+build_h := $(obj)/build.h
+
+# We have to manually export variables that `genbuild_h.sh` uses
+# when we call it through the `$(shell)` function. This is fragile
+# but as variables newly added to `genbuild_h.sh` would just not
+# work, we'd notice that instantly at least.
+build_h_exports := BUILD_TIMELESS KERNELVERSION COREBOOT_EXTRA_VERSION
+
+# Report new `build.ht` as dependency if `build.h` differs.
+build_h_check := \
+	export $(foreach exp,$(build_h_exports),$(exp)="$($(exp))"); \
+	util/genbuild_h/genbuild_h.sh >$(build_h)t 2>/dev/null; \
+	cmp -s $(build_h)t $(build_h) >/dev/null 2>&1 || echo $(build_h)t
+
+$(build_h): $$(shell $$(build_h_check))
 	@printf "    GEN        build.h\n"
-	rm -f $(obj)/build.h
-	util/genbuild_h/genbuild_h.sh > $(obj)/build.ht
-	mv $(obj)/build.ht $(obj)/build.h
+	mv $< $@
 
 build-dirs:
 	mkdir -p $(objcbfs) $(objgenerated)