Compile-time analysis may help us catch latent bugs in barebox. For this
to be most effective, we need an easy way to compile as much of barebox
as possible. Giving all driver options prompts would do this, but at the
cost of making user experience worse, by asking them about drivers for HW
that's clearly not relevant to the platform they selected.

Do as Linux does and provide a default-off COMPILE_TEST option, which we
can use to make extra drivers selectable.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>