util/genbuild_h: add COREBOOT_BUILD_EPOCH seconds since epoch

To use SOURCE_DATE_EPOCH for the kernel build, extend genbuild_h to
contain COREBOOT_BUILD_EPOCH.

Change-Id: Iaa79d3e7df8101a1ba1b37a361d8992f7eab2d52
Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51362
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/util/genbuild_h/genbuild_h.sh b/util/genbuild_h/genbuild_h.sh
index b82a74b..10ca0c5 100755
--- a/util/genbuild_h/genbuild_h.sh
+++ b/util/genbuild_h/genbuild_h.sh
@@ -68,6 +68,7 @@
 printf "#define COREBOOT_BUILD_MONTH_BCD 0x$(our_date "$DATE" +%m)\n"
 printf "#define COREBOOT_BUILD_DAY_BCD 0x$(our_date "$DATE" +%d)\n"
 printf "#define COREBOOT_BUILD_WEEKDAY_BCD 0x$(our_date "$DATE" +%w)\n"
+printf "#define COREBOOT_BUILD_EPOCH \"$(our_date "$DATE" +%s)\"\n"
 printf "#define COREBOOT_DMI_DATE \"$(our_date "$DATE" +%m/%d/%Y)\"\n"
 printf "\n"
 printf "#define COREBOOT_COMPILE_TIME \"$(our_date "$DATE" +%T)\"\n"