kbuild: add ability to make source rpm buildable using koji

Changes:
- added new target 'srcrpm-pkg' to generate source rpm
- added required build tools to spec file
- removed locally compiled host tools to force their re-compile

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: Jonathan Toppins <jtoppins@redhat.com>
Acked-by: Íñigo Huguet <ihuguet@redhat.com>
Tested-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>