cache: allow parsing a recipe with a custom config datastore

cache: allow parsing a recipe with a custom config datastore

To accommodate the OpenEmbedded layer index recipe parsing, we have to
have the ability to pass in a custom config datastore since it
constructs a synthetic one. To make this possible after the multi-config
changes, rename the internal _load_bbfile() function to parse_recipe(),
make it a function at the module level (since it doesn't actually need
to access any members of the class or instance) and move setting
__BBMULTICONFIG inside it since other code will expect that to be set.

Part of the fix for [YOCTO #10192].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>