tests/bsim/bt conn: Add support for split build with sysbuild

Add sysbuild build files, which allows building this test
both for the targets we could already before
(nrf52_bsim & nrf5340bsim_nrf5340_cpunet)
and also adds support for the a split build with the
nrf5340bsim_nrf5340_cpuapp.

Note that when doing a split build the controller is
taken from the hci_rpmsg BT sample, and therefore the
controller configuration in this folder is not used.

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