iso2usb.sh: allow custom parition sizing

Some people asked if it could be possible to reserve space on the USB stick
for a custom additional 4th partition, to be used freely and which will not get
touched by liveslak.

So, there is a new parameter to the script now, here is the help text:
  -y|--layout <x,x,x,x>      Specify partition layout and sizes (in MB).
                             Default values: '$DEF_LAYOUT' for 3 partitions,
                             the '-1' value for partition 3 meaning
                             'use all remaining space',
                             and an empty 4th value means 'do not reserve
                             free space for a custom 4th partition'.

The default layout of the USB stick remains:
  partition 1 (1MB),
  partition 2 (100 MB)
  partition 3 (claim all free space - specified as 0 MB).
The script allows for an amount of free space to be left at the end
(partition 4, unused by liveslak) in case you need this:
This translates to the following DEF_LAYOUT variable value:
DEF_LAYOUT="1,100,-1,"

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