Fixed having the root filesystem on USB when fstype was ext4.

This was caused by an too old kernel and/or the absense of /lib/modules
in the resulting image. By setting 'kernel_module' to true, the kernel
package will be installed and the various modules will be build.

This is not a complete fix, since the other filesystems don't
automatically work. If the fstype is btrfs, then adding btrfs-tools to
the packages option makes it work. With f2fs things are a bit trickier,
since there is a f2fs-tools package, but only for jessie, not wheezy.

But this will fix the default case (ext4) and in the meantime I'll work
on a better fix for the other filesystem types.