bundle: DRYer root.path entry
bundle: DRYer root.path entry

Instead of repeating the config.md conditions, just make the linkage
more clear.  Now the chain is:

1. Bundles REQUIRE a config.json which is a bundle artifact.
2. If that config.json has a root.path entry (as specified in
   config.md), then add the referenced directory to the set of bundle
   artifacts.  The config.md requirements include "If defined, a
   directory MUST exist at the path declared by the field".
3. Apply the "MUST all be present in a single directory" condition to
   all bundle artifacts.  I don't like that direct-child restriction
   [1], but I'm not touching it in this commit.

So these are the same requirements as before this commit, but with
less redundancy and fewer words.

[1]: https://github.com/opencontainers/runtime-spec/pull/469

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

opencontainers/runtime-spec,wking/opencontainer-runtime-spec,hqhq/specs,laijs/specs,wking/opencontainer-specs,hqhq/runtime-spec,opencontainers/specs