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>

Config.in

@@ -225,10 +225,12 @@ config BR2_BACKUP_SITE
 
 config BR2_KERNEL_MIRROR
 	string "Kernel.org mirror"
-	default "https://www.kernel.org/pub"
+	default "https://cdn.kernel.org/pub"
 	help
-	  kernel.org is mirrored on a number of servers around the world.
-	  The following allows you to select your preferred mirror.
+	  kernel.org is mirrored on a number of servers around the
+	  world.  The following allows you to select your preferred
+	  mirror. By default, a CDN is used, which automatically
+	  redirects to a mirror geographically close to you.
 
 	  Have a look on the kernel.org site for a list of mirrors, then enter
 	  the URL to the base directory.  Examples: