cmd: booti: move CONFIG_CMD_BOOTI to Kconfig

This command is used to boot ARM64 Linux.

I made DISTRO_DEFAULTS select this option for ARM64 to respect
include/config_distro_defaults.h.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Tom Rini <trini@konsulko.com>