[Buildroot] [PATCH] core/reproducible: do not override SOURCE_DATE_EPOCH

[Yann E. MORIN]

SOURCE_DATE_EPOCH is currently forcibly set (to either the git commit
date, or the last release date).

However, the spec mandates that it should not be modified if already
set: https://reproducible-builds.org/specs/source-date-epoch/

    Build systems MUST NOT overwrite this variable for child
    processes to consume if it is already present.

Abide by the rule, and only set it if not already set.

This will allow users to pass it from an upper-layer buildsystem (e.g. a
jenkins or gitlab-ci job, for example), when they have a reson to do so.

Reported-by: Peter Korsgaard <peter at korsgaard.com>
Reported-by: Einar Jón Gunnarsson <tolvupostur at gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998 at free.fr>
Cc: Peter Korsgaard <peter at korsgaard.com>
Cc: Einar Jón Gunnarsson <tolvupostur at gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
Cc: Arnout Vandecappelle <arnout at mind.be>
---
 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 79db7fe48a..7a727075ea 100644
--- a/Makefile
+++ b/Makefile
@@ -254,7 +254,7 @@ export LANG = C
 export LC_ALL = C
 export GZIP = -n
 BR2_VERSION_GIT_EPOCH = $(shell GIT_DIR=$(TOPDIR)/.git $(GIT) log -1 --format=%at)
-export SOURCE_DATE_EPOCH = $(if $(wildcard $(TOPDIR)/.git),$(BR2_VERSION_GIT_EPOCH),$(BR2_VERSION_EPOCH))
+export SOURCE_DATE_EPOCH ?= $(if $(wildcard $(TOPDIR)/.git),$(BR2_VERSION_GIT_EPOCH),$(BR2_VERSION_EPOCH))
 DEPENDENCIES_HOST_PREREQ += host-fakedate
 endif
 
-- 
2.11.0