[build] Use SOURCE_DATE_EPOCH if it exists

See https://reproducible-builds.org/docs/source-date-epoch/ for
rationale.

Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

@@ -1167,7 +1167,19 @@ BUILD_ID_CMD	:= perl -e 'printf "0x%08x", int ( rand ( 0xffffffff ) );'
 
 # Build timestamp
 #
+# Used as a means to automatically select the newest version of iPXE
+# if multiple iPXE drivers are loaded concurrently in a UEFI system.
+#
+# It gets rounded down to the nearest minute when used for this
+# purpose.
+#
+ifdef SOURCE_DATE_EPOCH
+BUILD_TIMESTAMP := $(SOURCE_DATE_EPOCH)
+else ifdef GITVERSION
+BUILD_TIMESTAMP := $(shell git log -1 --pretty=%ct)
+else
 BUILD_TIMESTAMP := $(shell date +%s)
+endif
 
 # Build version
 #