Makefile: always call date with LC_ALL=C set

Ensure that date is called only with LC_ALL=C locale set to make dates
locale neutral thus preventing lurking of non-ASCII characters into
U-Boot binary.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>

Changed LANG= into LC_ALL= as suggested by Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Wolfgang Denk <wd@denx.de>