Bluetooth: Kconfig: Restructure for a more logical hierarchy

Restructure the Bluetooth options more logically.

- Both host and controller are now behind the same high level
  CONFIG_BLUETOOTH.

- Selecting controller support disables other HCI driver selection, so
  the controller isn't in the same list as HCI drivers any more.

- Under the top-level there's a "Custom stack" option, which when
  enabled opens up the option of choosing CONFIG_NBLE.

There are various other cleanups and simplifications in this patch as
well, since splitting these up would have been fairly tricky while
making sure all test cases still build.

Change-Id: I5bb715cb9d20201cb8b72fbd149c8a09a4b2d7d2
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>