snippets: Add snippet for the Zephyr Bluetooth Controller

By defining this snippet it becomes simpler to build an application
for the Zephyr Bluetooth Controller in environments
where this is not enabled in device tree by default.

It removes the need for adding boilerplate overlay files
to applications.

Also, we would be able to add this snippet by default for
samples and tests where it is required to use the zephyr
link layer.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>