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
Merge "Parameterise PXE kernel and initrd selection"