firmware-utils: honor env SOURCE_DATE_EPOCH

Use the timestamp from the enviroment SOURCE_DATE_EPOCH
if set instead of the build time.
Fixes reproducible builds for certain firmware images.

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

tools/firmware-utils/src/zyxbcm.c

@@ -23,7 +23,6 @@
 #include <stdlib.h>
 #include <string.h>
 #include <stdint.h>
-#include <time.h>
 #include <unistd.h>
 #include <sys/stat.h>