mkext3: add --skip-grub option

mkext3: add --skip-grub option

For buildling debxo images (or other non-x86 images), we don't care about
installing grub.  Furthermore, on some platforms, we don't have the grub
command.. so requiring grub to generate images is a non-starter.

The long-term solution to this is to have something like:
 --use-bootloader=grub
...where one can use that to select between grub, ofw, uboot, etc.

Signed-off-by: Andres Salomon <dilinger@queued.net>