firmware_class: Introduce a firmware descriptor structure

Introduce a firmware descriptor structure that makes it
easier to pass around various configuration options in the
internal implementation of firmware_class.

Change-Id: I5c1da222bccd568fabb26da5baccaa4035331efd
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>
[joshc: reworked to make use of upstream's opt_flags member]
Signed-off-by: Josh Cartwright <joshc@codeaurora.org>
[vmulukut: adjusted for upstream changes to opt_flags]
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>