board/raspberrypi: use kernel Bluetooth mode

board/raspberrypi: use kernel Bluetooth mode

The default setting with miniuart-bt requires hciattach which is a
deprecated utility in BlueZ.  Setting the krnbt parameter switches to
the modern method of using serdev in the kernel removing the need for
any userspace configuration to enable the Bluetooth controller.

This is documented as applying to all Raspberry Pi variants so just
enable it globally.

Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>