sdk: predefine SOURCE_DATE_EPOCH

When building packages within the SDK, there is no Git revision history
available so prepopulate SOURCE_DATE_EPOCH in version.mk, similar to
how we handle REVISION already.

Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

include/toplevel.mk

@@ -16,13 +16,14 @@ ifeq ($(SDK),1)
   include $(TOPDIR)/include/version.mk
 else
   REVISION:=$(shell $(TOPDIR)/scripts/getver.sh)
+  SOURCE_DATE_EPOCH:=$(shell $(TOPDIR)/scripts/get_source_date_epoch.sh)
 endif
 
 HOSTCC ?= $(CC)
 export REVISION
+export SOURCE_DATE_EPOCH
 export GIT_CONFIG_PARAMETERS='core.autocrlf=false'
 export MAKE_JOBSERVER=$(filter --jobserver%,$(MAKEFLAGS))
-export SOURCE_DATE_EPOCH:=$(shell $(TOPDIR)/scripts/get_source_date_epoch.sh)
 
 # prevent perforce from messing with the patch utility
 unexport P4PORT P4USER P4CONFIG P4CLIENT

include/version.mk

@@ -8,6 +8,7 @@
 
 # Substituted by SDK, do not remove
 # REVISION:=x
+# SOURCE_DATE_EPOCH:=x
 
 RELEASE:=Reboot
 

target/sdk/Makefile

@@ -118,6 +118,7 @@ $(BIN_DIR)/$(SDK_NAME).tar.bz2: clean
 		cat $(TOPDIR)/feeds.conf.default >> $(SDK_BUILD_DIR)/feeds.conf.default; \
 	fi
 	$(SED) 's,^# REVISION:=.*,REVISION:=$(REVISION),g' $(SDK_BUILD_DIR)/include/version.mk
+	$(SED) 's,^# SOURCE_DATE_EPOCH:=.*,SOURCE_DATE_EPOCH:=$(SOURCE_DATE_EPOCH),g' $(SDK_BUILD_DIR)/include/version.mk
 	$(SED) '/LINUX_VERMAGIC:=/ { s,unknown,$(LINUX_VERMAGIC),g }' $(SDK_BUILD_DIR)/include/kernel.mk
 	find $(SDK_BUILD_DIR) -name .git | $(XARGS) rm -rf
 	find $(SDK_BUILD_DIR) -name .svn | $(XARGS) rm -rf