Debian: Support additional debootstrap arguments

Add support for supplying a custom keyring and debootstrap script
using two new environment variables:

* DIB_DEBIAN_KEYRING - Path to a gpg keyring.
* DIB_DEBIAN_DEBOOTSTRAP_SCRIPT - Path to a alternative debootstrap
  script.

Change-Id: I42e8832b435b8a77b0681b5bc9d9332bbe860cd5
Parameterise PXE kernel and initrd selection

When using custom-built kernels, their filenames may not necessarily
follow the standard naming conventions for the stock systems in question.

This patch permits the specification of an alternative pattern to use
when looking for the correct kernel to construct the PXE images from.

Example:
  export DIB_BAREMETAL_KERNEL_PATTERN="kernel*jang"
  export DIB_BAREMETAL_INITRD_PATTERN="initrd*jang"

Change-Id: I86e5254051addd5e89dd4e363e9bba117e65353d
Tidy up SuSE kernel selection

This is just removing a C&P line that only ever returns a single result.

Change-Id: Iad37e982412a9199d86d9b4481cc14a85eef704e
Merge "Parameterise PXE kernel and initrd selection"
Merge "Tidy up SuSE kernel selection"
Merge "Debian: Support additional debootstrap arguments"