Implement support for flashing sparse images

[obbardc]

There are a number of open PRs trying to implement sparse image format:

• Implement sparse image support (no deps on libsparse) #37
• Implement sparse image support #30
• Implement sparse image flashing (yet another) #6
• Sparse image format + Some fixes #5

It'd be really nice to get something merged which implements this and close out the other PRs :-)

[obbardc]

cc @lumag @ndechesne @andersson ; not sure if any of you are interested in this feature?

[andersson]

@obbardc it does sound like a good feature, and the number of PRs just confirms that there is user demand.

I didn't like the initial attempts due to their added dependencies.
I think #37 looks good implementation wise, and I wanted to dig into this further, but I don't have any sparse use cases and my attempt at trying it out didn't give me the expected results (perhaps due to user error, perhaps because we don't have erase-before-flash support?), and I didn't invest the necessary time to figure out why.

I'd be happy to see #37 move forward.