fix up EC2 image creation with newer versions of mkinitrd

- improve EC2 initrd creation
  + consider filesystem used, do not blindly assume ext3
    (latent unreported bug)
  + create a "monster" initrd to avoid the mkinitrd code from probing
    into the device tree. The device tree is different on the build
    machine than the target EC2 infrastructure
- exit early if no EC2 account credentials are defined
  + if no credentials are defined we cannot create a bundle, exit after
    initrd creation if no credentials exist