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>

Makefile

@@ -1278,6 +1278,7 @@ define filechk_timestamp.h
 			LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_DATE "%b %d %C%y"'; \
 			LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_TIME "%T"'; \
 			LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_TZ "%z"'; \
+			LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_DMI_DATE "%m/%d/%Y"'; \
 		else \
 			return 42; \
 		fi; \
@@ -1285,6 +1286,7 @@ define filechk_timestamp.h
 		LC_ALL=C date +'#define U_BOOT_DATE "%b %d %C%y"'; \
 		LC_ALL=C date +'#define U_BOOT_TIME "%T"'; \
 		LC_ALL=C date +'#define U_BOOT_TZ "%z"'; \
+		LC_ALL=C date +'#define U_BOOT_DMI_DATE "%m/%d/%Y"'; \
 	fi)
 endef