linux: add support for 3.x and -rc versions

The assumption that all kernels are in
http://www.kernel.org/pub/linux/kernel/v2.6/

 is no longer true:
versions 3.x are in separate directories.

We now compute the directory name from the major and minor versions of
the version provided by the user. This assumes that the 3.1 version
will be in a /v3.1/ directory, which we don't know yet because the 3.1
cycle hasn't started yet.

At the same time, we add support for the official -rcX versions.

Patch tested by compiling 3.0-rc6, which Buildroot has successfully
downloaded and built.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>