sandbox: Rework how SDL is enabled / disabled

Given that we can use Kconfig logic directly to see if we have a program
available on the host or not, change from passing NO_SDL to instead
controlling CONFIG_SANDBOX_SDL in Kconfig directly. Introduce
CONFIG_HOST_HAS_SDL as the way to test for sdl2-config and default
CONFIG_SANDBOX_SDL on if we have that, or not.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>