linux-libc-headers: introduce 4.9 headers

linux-libc-headers: introduce 4.9 headers

Bumping the linux-libc-headers to the 4.9 kernel variant. Since 4.9
will be a LTSI kernel, and there will be an associated linux-yocto
it is worth moving to these headers sooner rather than later.

This commit also drops the 4.8 headers and sets 4.9 to be the default.

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>