buildroot

Config.in: Use CDN kernel.org mirror

It turned out for quite some time CDN-powered mirror
of kernel.org is available. See this announce:
https://www.kernel.org/introducing-fastly-cdn.html

This change switches to that faster mirror by default.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
[Thomas: tweak the help text.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>