Prepare disk image in generate_boot_image.sh

* instead of directly formatting and partitioning an SDcard,
  generate_boot_image now prepares a disk image file
* setup via losetup, formatted and mounted via kpartx
* much faster than previous method, including faster writing to SD
* major stylistic improvements, clearer style
* fixed bug in parallel executions

boot/misc/sfdisk.script

+2048 204800 c, *
+206848 7655424 83 -