[PATCH v2 0/5][disco] Add support for UEFI signed kernels on arm64

[dann frazier]

BugLink: https://bugs.launchpad.net/bugs/1804481

The following patches add support for signed UEFI kernel images on
arm64. The first three patches are for the linux package and the last
two are for linux-signed.

The patches are complicated a bit by the fact that our arm64 generic
kernels are gzip compressed. We wish to keep the kernels we install
compressed both in the linux-image and linux-image-unsigned packages,
however signing must be done on the uncompressed kernel image. Therefore
we decompress the kernel when adding it to the signing tarball and bundle
a configuration file to signal linux-signed to recompress.

Test builds are available here:
  https://launchpad.net/~dannf/+archive/ubuntu/arm64-signed

v2:
  - Add support for a <efi-image>.vars config in the signed tarball,
    and support a GZIP=1 setting to tell linux-signed that the signed
    image should be recompressed.
  - Use maximum gzip compression when recompressing, to match the
    unsigned image.
  - Include snapdragon flavor support.
  - Kill the cat.