Since the SPD823TS board does not actually have any writable flash to save
its environment, undefine CONFIG_CMD_ENV so the "saveenv" command is
disabled.

This fixes the build error:
common/libcommon.a(cmd_nvedit.o): In function `do_saveenv':
common/cmd_nvedit.c:557: undefined reference to `saveenv'
make: *** [u-boot] Error 1

Signed-off-by: Mike Frysinger <vapier@gentoo.org>