Config.in: default to HTTPS for s.b.n backup site

Now that we have HTTPS support for sources.buildroot.net (through Lets
encrypt / Cloudflare), it makes sense to default to it for our backup site.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 05296ced369bab8877efa624f3d9b4d201ba5b38)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Config.in

@@ -241,7 +241,7 @@ if !BR2_PRIMARY_SITE_ONLY
 
 config BR2_BACKUP_SITE
 	string "Backup download site"
-	default "http://sources.buildroot.net"
+	default "https://sources.buildroot.net"
 	help
 	  Backup site to download from. If this option is set then
 	  buildroot will fall back to download package sources from here