bluetooth.bbclass: simplify recipe inference of bluetooth provider

bluetooth.bbclass: simplify recipe inference of bluetooth provider

Defines ${BLUEZ} to be the provider of bluez services as specified by
DISTRO_FEATURES.

Example recipe fragment:
  inherit bluetooth
  PACKAGECONFIG ??= "${@bb.utils.contains('DISTRO_FEATURES', 'bluetooth', '${BLUEZ}', '', d)}
  PACKAGECONFIG[bluez4] = "--enable-bluetooth,--disable-bluetooth,bluez4"
  PACKAGECONFIG[bluez5] = "--enable-bluez5,--disable-bluez5,bluez5"

[YOCTO #5031]

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>