config: Define BOOTP client architecture and VCI for ARMv8

Reuse the 32-bit ARM client architecture and identify ARMv8 specifically
by setting the BOOTP VCI string.

Cc: Dennis Gilmore <dennis@ausil.us>
Cc: Tom Rini <trini@konsulko.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>