buildroot

legacy/kernel-headers: adjust recommended target

Previously they were set at next available version, which is problematic
since new syscalls can be introduced.
So switch to lower available.
There's a caveat for 3.1 and 3.0 headers though since there's no lower
than 3.2, so keep it at 3.2 for those unfortunately.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>