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>

Makefile

@@ -1358,6 +1358,7 @@ define filechk_timestamp.h
 			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"'; \
+			LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_BUILD_DATE 0x%Y%m%d'; \
 		else \
 			return 42; \
 		fi; \
@@ -1366,6 +1367,7 @@ define filechk_timestamp.h
 		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"'; \
+		LC_ALL=C date +'#define U_BOOT_BUILD_DATE 0x%Y%m%d'; \
 	fi)
 endef