tests: Bluetooth: split `id_addr_update` test

Split FW into two images, one for central one for peripheral.

The SoftDevice controller's connection contexts are not role-agnostic:
some are reserved for the peripheral roles some for the central roles.

Splitting the test into two images is anyway "The right thing (TM)",
it's just that we sometimes take the shortcut of using a single image to
go faster.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>