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>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/build/tools.rst b/doc/build/tools.rst
index c06f915..ec017229 100644
--- a/doc/build/tools.rst
+++ b/doc/build/tools.rst
@@ -44,4 +44,4 @@
 Launch the MSYS2 shell of the MSYS2 environment, and do the following::
 
    $ make tools-only_defconfig
-   $ make tools-only NO_SDL=1
+   $ make tools-only