wic: Extend indirect string connection to support image names and rootfs

wic: Extend indirect string connection to support image names and rootfs

The wic command-line param --rootfs-dir gets extended to support
multiple directories and image names. Each '--rootfs-dir' could be
connected using a special string. This special string is a image
name which gets expanded by bitbake to get 'IMAGE_ROOTFS' or
a rootfs-dir path pointing to rootfs directory. Like this:

  wic create ... --rootfs-dir rootfs1=core-image-minimal \
    --rootfs-dir rootfs2=core-image-minimal-dev

.wks:

  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

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