config: Use booti instead of bootz on 64-bit ARM

The bootz command doesn't work with Linux kernel images on 64-bit ARM.
The replacement command with the same interface and functionality is
booti.

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