Using __DATE__ and __TIME__ results in an error due to -Werror=date-time
with gcc-4.9 (__DATE__ / __TIME__ might prevent reproducible builds) so
switch these over to U_BOOT_DATE / U_BOOT_TIME

Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Simon Glass <sjg@chromium.org>