bq-picmt: generate sparse rootfs, correct some bootimg offsets

Generating a sparse rootfs speeds up flashing through Fastboot. Also change some bootimg offsets to be more accurate to the one actually used by the bootloader. (They will actually just be ignored, since the bootloader always uses its own hard-coded ones.)

Tested that bq-picmt can be also used with the Cyanogen bootloader or lk2nd (too lazy to switch all the time in the future if I want to test something on downstream...) This already worked before, these changes just make it a bit more comfortable.

Cc: @lazzardo