wic: Extend --rootfs-dir to connect rootfs-dirs

wic: Extend --rootfs-dir to connect rootfs-dirs

The wic command-line param --rootfs-dir gets generalized to support
multiple directories. Each '--rootfs-dir' could be connected using a
special string, that should be present in .wks. I.e:

wic create ... --rootfs-dir rootfs1=/some/rootfs/dir \
  --rootfs-dir rootfs2=/some/other/rootfs/dir

  part / --source rootfs --rootfs-dir="rootfs1" --ondisk sda --fstype=ext3 \
    --label primary --align 1024

  part /standby --source rootfs --rootfs-dir="rootfs2" \
    --ondisk sda --fstype=ext3 --label secondary --align 1024

The user could use harded-code directory instead of connectors. Like this:

  wic create ... hard-coded-path.wks -r /some/rootfs/dir

  part / --source rootfs --ondisk sda --fstype=ext3 --label primary --align 1024

  part /standby --source rootfs --rootfs-dir=/some/rootfs/dir \
    --ondisk sda --fstype=ext3 --label secondary --align 1024

Signed-off-by: João Henrique Ferreira de Freitas <joaohf@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>