Really no need to test and sleep when there's sync mode.

Factor of 2 is not enough for kernel upgrade due to filesystem overhead.
Add BOOT_SIZE_FACTOR on top of that.

Overview of the changes:
- ARM support: separate ext2 /boot, no LILO
- avoid race condition with devmapper
- trap ERR so that -e in shebang doesn't result in extra cleanup hassle
- configurable root filesystem type (ext4 by default)
- jumps through parted hoops

Details:

1. LILO is x86-specific while the rest of the script can be used
   to prepare e.g. Marvell ArmadaXP or CuBox images; we can generally
   count on uboot supporting ext2 for relatively sane platforms but
   not ext4 that would be a better root filesystem performance-wise.

2. Apparently /dev/mapper/loopXpY can be still missing at the time
   when kpartx returns and pop up a bit later... sit there, wait
   and check for it.

3. If something went wrong with any command of the script it would bail out
   due to -e in shebang; it is now better to clean up the loopback device
   and its mappings in this situation either.

4. One size doesn't fit all, really.

5. The parted sizing was sloppy as in broken, now it's just half insane.
   Someone's decision to stick units and auto-alignment knobs into
   a single one was apparently hilarious...

   http://www.gnu.org/software/parted/manual/parted.html#unit

Manual loop/dm cleanup is described in documentation just in case.

/boot size meter is suboptimal in terms of additional I/O incurred,
will be most likely rewritten to make use of advance "du -s".