coredump: Generalize the shell module

This might come in handy for other coredump backends too, not only flash
one. Obviously: changing the logic to use the coredump generic API instead
of the flash backend's internal functions.

Adding a better output, following how the coredump is actually created:
parsing and printing headers, and their respective content.

Signed-off-by: Tomasz Bursztyka <tobu@bang-olufsen.dk>