With device trees, boards do not always set CONFIG_MACH_TYPE now, so we
must not rely on this define being set.  The kernel uses ~0 to see if we
have a valid machine number or not, so set that as the default, invalid
machine, id and only fix if CONFIG_MACH_TYPE is set.

Acked-by: Dan Murphy <dmurphy@ti.com>
Tested-by: Heiko Schocher <hs@denx.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Tom Rini <trini@ti.com>