cassini-[bsp,ci,config,docs]: Add layer to hold board specific packages

Adds meta-cassini-bsp layer to hold board specific recipes and append
files which have not been (or will not be) upstreamed.

Layer added to base kas config, QA checks, and documentation

By using BBFILES_DYNAMIC, meta-cassin-bsp only adds recipes/append files
for platforms which are present

Changlog: other
Signed-off-by: Adam Johnston <adam.johnston@arm.com>