wic: extend empty plugin with options to write zeros to partiton

wic: extend empty plugin with options to write zeros to partiton

Adds features to explicitly write zeros to the start of the
partition. This is useful to overwrite old content like
filesystem signatures which may be re-recognized otherwise.

The new features can be enabled with
'--soucreparams="[fill|size=<N>[S|s|K|k|M|G]][,][bs=<N>[S|s|K|k|M|G]]"'
Conflicting or missing options throw errors.

The features are:
- fill
  Fill the entire partition with zeros. Requires '--fixed-size' option
  to be set.
- size=<N>[S|s|K|k|M|G]
  Set the first N bytes of the partition to zero. Default unit is 'K'.
- bs=<N>[S|s|K|k|M|G]
  Write at most N bytes at a time during source file creation.
  Defaults to '1M'. Default unit is 'K'.

Signed-off-by: Malte Schmidt <malte.schmidt@weidmueller.com>
Signed-off-by: Lukas Funke <lukas.funke@weidmueller.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>