sparse: Implement storage abstraction

The current sparse image parser relies heavily on the MMC layer, and
doesn't allow any other kind of storage medium to be used.

Rework the parser to support any kind of storage medium, as long as there
is an implementation for it.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>