sparse: Rename the file and header

The Android sparse image format is currently supported through a file
called aboot, which isn't really such a great name, since the sparse image
format is only used for transferring data with fastboot.

Rename the file and header to a file called "sparse", which also makes it
consistent with the header defining the image structures.

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