Bluetooth HCI driver location & Kconfig restructuring

[jhedberg]

This primarily affects local Link Layer drivers, but the first bullet relates to all HCI drivers.

• We only have one type of Bluetooth drivers, namely HCI drivers, so the drivers should move from drivers/bluetooth/hci/ to simply drivers/bluetooth/. This is also in line with the fact that the corresponding header file is include/zephyr/drivers/bluetooth.h.
• BT_CTLR should be, as per its definition, used for all cases where you have a local LL. Until now only Nordic has taken advantage of it, both for the upstream native LL as well as their downstream SD LL.
  1. Remove BT_LL_CHOICE
  2. Make BT_CTLR a virtual option that's selected by users (like BT_LL_SW_SPLIT or any other "local controller" HCI driver)
  3. Use default y for anything that depends on a status = "okay"; in DT (like BT_LL_SW_SPLIT)
  4. Remove any current explicit enabling of CONFIG_BT_CTLR=y in prj.conf files, since it's now auto-enabled based on DT. Explicit CONFIG_BT_CTLR=n may be necessary in some places.
  5. Remove this redundant pattern which seems to exist in many Kconfig.defconfig files:

   config BT_CTLR
   	default BT
  

  The reason it's pointless is that BT_CTLR already depends on BT and now that it'll get automatically selected there's no point in doing these tricks anymore.

[jhedberg]

Updated the description with my latest thoughts/plan

[jhedberg]

I'll close this as most of it was already implemented by #82157 and the rest will be likely done differently than what this issue describes (e.g. it looks like the controller driver doesn't need to move anywhere)