Depending on the types of deployment (security, nfv...) some extra
kernel flags are needed on the images. This change exposes the
DIB_BOOTLOADER_DEFAULT_CMDLINE parameter, defaulting to the
existing 'nofb nomodeset vga=normal', that will allow to modify
these boot params.
Change-Id: I67d191fa5ca44a57f776cb9739a02dd71212969c
Closes-Bug: #1668890