fastboot: add support for auto-resparsing large files

Add support to fastboot for automatically using libsparse to break large
files, whether they are in sparse or normal format, into multiple sparse
files that can each fit into the target's memory.  Allows flashing
images that are larger than the size of the available memory on the
target.

By default, any file over 512MB will be sparsed into 512MB chunks.  The
limit can be modified with the -m argument, or sparsing can be forced
with -S or avoided with -N.  If -m is not specified, the target can
override the default by implementing getvar:max-download-size

Change-Id: I6c59381c3d24475c4f2587ea877200b96971cbd7