Usage: cm-chroot-sw-img <image-directory> [command [args...]]

Change root into a BCM software image directory (normally /cm/images/<name>)
after bind-mounting /proc, /sys and /dev, run the command (or a shell when
none is given), then unmount everything again.

Use it to install or remove packages inside an image, for example:
  cm-chroot-sw-img /cm/images/<image> apt-get install -y <package>

Changes reach running nodes only after an image update (cmsh: device;
imageupdate) or a reboot that provisions them from the image.
