Bluetooth: controller: Add BT_CTLR_HCI Kconfig option

[jhedberg]

Add a new option to split off the building of the HCI layer of the controller. This layer is not needed by the controller unit tests, and becomes problematic with the planned change to use devicetree for HCI drivers (causing CI failures in #72323).

[jhedberg]

@tejlmand any idea what the Compliance failure about a non-existing BOARD_UNIT_TESTING option is about? When I built locally I could verify that it does exist. In fact I have these two in build/zephyr/.config when I build a controller unit test:

CONFIG_BOARD_UNIT_TESTING=y
CONFIG_BOARD_UNIT_TESTING_UNIT_TESTING=y

[tejlmand]

@tejlmand any idea what the Compliance failure about a non-existing BOARD_UNIT_TESTING option is about?

because when running compliance on the Kconfig tree, then only the pure Zephyr tree is used and hence the Kconfig parts of the unit testing board is not sourced.

One reason is to avoid such test settings leaking into the production code 😉

[tejlmand]

see proposal here: https://discord.com/channels/720317445772017664/1240262352427810888/1240648479504601119

[cvinayak]

Use "Bluetooth: Controller:..." in commit title (BT Spec no longer uses lower case controller in its texts)

[jhedberg]

Use "Bluetooth: Controller:..." in commit title (BT Spec no longer uses lower case controller in its texts)

Done.