Run flash-kernel on all platforms and update-initramfs on ls1028a

See merge request !82

v2: unify the image creation into a single script sharing the same rootfs

See merge request !81

System Image V4, Introduce reform2-a311d

See merge request !75

don't write /etc/initramfs-tools/modules as reform-tools 1.25 ships...

See merge request !77

don't write /etc/initramfs-tools/modules as reform-tools 1.25 ships /usr/share/initramfs-tools/modules.d/reform.conf

Use disk label instead of /dev paths

See merge request !76

 - enables using the same /etc/fstab for imx and a311d where /dev/mmcblk
   order is switched
 - enables flashing the image to a usb-stick for which /dev/mmcblk*
   would also be wrong
 - makes the created images more compatible with debvm-run which
   requires a disk label to be set