Handle non-existing *generic kernel and initrd

Finding the Debian boot kernel and initrd is collapsed into a
single statement so that the script doesn't bail out when
'set -o pipefail' is set and the *generic kernel and initrd are
absent.

Change-Id: Ifd616818f7387e6f102636c7cf186be6097f1d5c
Closes-Bug: 1335016
Merge "Handle non-existing *generic kernel and initrd"