kernel headers: remove deprecated version 3.8

Kernel headers version 3.8.x has been deprecated since 2013.08 and thus can
be removed in 2014.08.
An automatic selection of 3.9.x headers is performed in the legacy menu.
Existing automatic selections of 3.8.x headers are modified to select
3.9.x.

As this patch removes the last occurrence of BR2_DEPRECATED_SINCE_2013_08,
the symbol is removed too.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>