sparse: Move main header parsing to a function of its own

The current sparse image format parser is quite tangled, with a lot of
code duplication.

Start refactoring it by moving the header parsing function to a function
of its own.

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