kernel headers: remove deprecated versions 3.6 and 3.7

Kernel headers versions 3.6.x and 3.7.x have been deprecated since 2013.05
and thus can be removed in 2014.05.
An automatic selection of 3.8.x headers is performed in the legacy menu.
An existing automatic selection of 3.6.x headers is modified to select
3.8.x.

As this patch removes the last occurrances of BR2_DEPRECATED_SINCE_2013_05,
the symbol is removed too.

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