kernel-artifact-names.bbclass, kernel.bbclass: remove prefix and extension from MODUL...

kernel-artifact-names.bbclass, kernel.bbclass: remove prefix and extension from MODULE_TARBALL_* variables

* for consistency with other artifacts variables, include only the version string, not the actual name or extension
* changing .tgz to something else in the MODULE_TARBALL_NAME variable only wouldn't make much sense
  because then kernel.bbclass still calls "tar -cvzf" to create it

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>