sanitycheck: infer ZEPHYR_BASE if unset

Now that the main Zephyr build system (and the documentation's) no
longer require ZEPHYR_BASE in the environment, the sanitycheck
script's continuing to require this makes less sense.

We can easily find ZEPHYR_BASE starting from a given sanitycheck
script, so let's just do that. Preserve the ability for a user to
override the ZEPHYR_BASE location in the environment as usual for
compatibility.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>