tests bsim/bt/ll: Split build script in 2

The CIS tests are building quite many
apps on their own.
Let's split them in their separate sub-script,
so we don't parallelize too many builds,
and users have more granularity if they only
want to build a subset.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>