Makefile: release: make manual build reproducible

The manual contains build timestamps like:

Buildroot 2025.08-rc1 manual generated on 2025-08-12 06:43:37 UTC

Making it not reproducible.  Luckily asciidoc respects SOURCE_DATE_EPOCH, so
use that to use the timestamp of the latest git commit instead.

Notice: The release and manual targets are marked as noconfig targets (and
need to stay like that for make release), so we cannot use the
BR2_REPRODUCIBLE logic.  Instead manually set SOURCE_DATE_EPOCH.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

Makefile

@@ -1250,7 +1250,8 @@ release: OUT = buildroot-$(BR2_VERSION)
 # documentation to the git output
 release:
 	git archive --format=tar --prefix=$(OUT)/ HEAD > $(OUT).tar
-	$(MAKE) O=$(OUT) manual-html manual-text manual-pdf
+	SOURCE_DATE_EPOCH=$$(git log -1 --format=%at 2> /dev/null) \
+		$(MAKE) O=$(OUT) manual-html manual-text manual-pdf
 	$(MAKE) O=$(OUT) distclean
 	tar rf $(OUT).tar $(OUT)
 	gzip -9 -c < $(OUT).tar > $(OUT).tar.gz