uboot-phytec

Makefile: Export build date as integer

In some cases we would need build date as integer value.
Export U_BOOT_BUILD_DATE as %Y%m%d integer value.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>