greybus: manifest: rework bundle parsing

Rework the the code that parses the manifest for bundles so it only
touches each manifest descriptor once.  (Previously the list was
scanned from the beginning repeatedly until all bundles were found.)
Shorten the name of the descriptor variable, to avoid line wrap.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>