tests/bsim/bt/host: Split build script in 6

There are quite many BT host test images being built.
Today these are all built in parallel, causing a quite
high load.

Let's split them in their separate sub-scripts,
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>