wic: Use partition label to be part of rootfs filename

wic: Use partition label to be part of rootfs filename

When a partition from .wks file is set up like this:

  part /standby --source rootfs --rootfs-dir=<special rootfs> ... --label \
    --label secondary

This means that 'rootfs' must use '<special rootfs>' as rootfs and
the default partition filename in /var/tmp/wic/build/ will be create
using the '--label' as part of the name. E.g:

  /var/tmp/wic/build/rootfs_secondary.ext3

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