Bluetooth: Permit connectionless host-controller combined build

In a host plus controller combined build, if no connection
is required, deselect CONFIG_BLUETOOTH_MAX_CONN. This will
reduce RAM and ROM usage in the controller.

Also, make BLUETOOTH_PERIPHERAL, BLUETOOTH_CENTRAL and
BLUETOOTH_CONN switches accessible by the controller kconfig
to select the right roles to enable.

Change-id: I164cf696ab2a6f4859086d2cb18f6d3f2b1399d3
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>