Makefile: Generate U_BOOT_DMI_DATE for SMBIOS

Add U_BOOT_DMI_DATE (format mm/dd/yyyy) generation to be used by
SMBIOS tables, as required by SMBIOS spec 3.0 [1]. See chapter 7.1,
BIOS information structure offset 08h for details.

[1] http://www.dmtf.org/sites/default/files/standards/documents/DSP0134_3.0.0.pdf



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