Add support for sparse ext4 image creation.

This adds the -s option to the make_ext4fs tool, which now creates
"sparse" filesystem images, which is very useful for the large 32 Gbyte
filesystems we are now building.
This check-in also fixes make_ext4fs to properly create filesystems
larger thatn 4 Gbytes on 64-bit Linux, 32-bit android and Macs.

Change-Id: Ie5838492fcf944f5c875481693c0dbd7013deae4
11 files changed