Kconfig: DISTRO_DEFAULTS: Only enable CMD_BOOTZ for ARM

The 'bootz' command is really only for ARM32 Linux Kernel 'zImage' files
but has also been adapted for testing with sandbox.  Given that sandbox
is a test platform, don't add that logic under DISTRO_DEFAULTS.

Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tom Rini <trini@konsulko.com>