[SRU bionic] linux-buildinfo support

[Andy Whitcroft]

Add linux-buildinfo package support.  This package contains all of the
ABI information accumulated during the build.  This means we no longer
have to install that data onto the end-user system, and means we only 
have to download the ABI data when doing getabis.

Pull request below.

Proposing for SRU to bionic.

-apw

The following changes since commit 9b820c951b75fbae2d2313377b70107a3e7f49fa:

  ALSA: rawmidi: Change resized buffers atomically (2018-11-29 02:41:15 -0500)

are available in the Git repository at:

  git://git.launchpad.net/~apw/ubuntu/+source/linux/+git/bionic linux-buildinfo

for you to fetch changes up to 4e1ec0a0b1cbebb60e07424eaffbdc768e0dd535:

  UBUNTU: [Config] buildinfo -- add retpoline version markers (2018-12-03 12:21:06 +0000)

----------------------------------------------------------------
  * linux-buildinfo: pull out ABI information into its own package
    (LP: #1806380)
    - [Packaging] limit preparation to linux-libc-dev in headers
    - [Packaging] commonise debhelper invocation
    - [Packaging] ABI -- accumulate abi information at the end of the build
    - [Packaging] buildinfo -- add basic build information
    - [Packaging] buildinfo -- add firmware information to the flavour ABI
    - [Packaging] buildinfo -- add compiler information to the flavour ABI
    - [Packaging] buildinfo -- add buildinfo support to getabis
    - [Packaging] buildinfo -- switch getabis configuration to new package
    - [Config] buildinfo -- add retpoline version markers

  * linux packages should own /usr/lib/linux/triggers (LP: #1770256)
    - [Packaging] own /usr/lib/linux/triggers