linux-yocto: introduce 4.19 recipes

linux-yocto: introduce 4.19 recipes

Introduce the 4.19 linux kernel as the reference for LTS and eventually
LSB distros. For now, we keep the other versions around until the h/w
reference BSPs are updated and 4.20+ are available as the "latest"
kernel. This avoid updating qemu* to 4.19 as the default for a small
transition window.

(From OE-Core rev: d52d68adff8fc2b9b2a98405f57fb06d2ed26c24)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>