linux/Config.in: clarify longterm 2.6 URL exception

As this question has popped up multiple times on the mailing list, clarify
that selecting longterm 2.6 kernels requires the 'custom tarball' option
instead of 'custom version'.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>