{linux, linux-headers}: default to 4.19.x

{linux, linux-headers}: default to 4.19.x

4.20.x is not a long term support kernel, but 4.19.x is (supported until end
2020):
https://www.kernel.org/category/releases.html

With the upcoming Buildroot 2019.02 release being a LTS release, default to
4.19.x instead.

Notice: The userspace API breakage in net_stamp.h causing build failures has
now been fixed in 4.19.14 by commit e4a2ffe9029fd (net: Use
__kernel_clockid_t in uapi net_stamp.h)

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
[Thomas: add comment in linux/Config.in and
package/linux-headers/Config.in.host so that we don't mistakenly bump
to 4.20+.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>