This revert back the commit 991a5c69

.
This causes more problems than solving (fixing the warning issue in parted)

With the default offset 0:
* makes posibble to create others extra partitions on USB key
  and use them inside the live enviroment.
* makes posible to use /dev/disk/by-label/ symlinks.
  archiso_early hook, can be removed.

Note: archboot also uses the default offset 0.

More info at:
http://mailman.archlinux.org/pipermail/arch-releng/2010-March/000912.html
http://mailman.archlinux.org/pipermail/arch-releng/2010-March/000890.html

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>