linux-raspberrypi: Switch default back to 4.14.y
linux-raspberrypi: Switch default back to 4.14.y

The 4.19.y kernel is not yet considered stable upstream as discussed in
the following GitHub issues:

https://github.com/agherzan/meta-raspberrypi/issues/410
https://github.com/raspberrypi/linux/issues/2931

Signed-off-by: Paul Barker <a027184a55211cd23e3f3094f1fdc728df5e0500@betafive.co.uk>

leon-anavi/meta-raspberrypi,agherzan/meta-raspberrypi,leon-anavi/meta-raspberrypi,schnitzeltony/meta-raspberrypi,schnitzeltony/meta-raspberrypi,leon-anavi/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,schnitzeltony/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi