spirv-tools: cleanup

- spirv-headers is provived in a separete receipe, so add it as dependencie
- add a new package spirv-tools-lesspipe that needs bash
- remove unneeded external sources

Signed-off-by: Jose Quaresma <quaresma.jose@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>