[PATCH 0/2][SRU][Disco] Add nvidia-418 dkms build support

[Seth Forshee]

BugLink: https://bugs.launchpad.net/bugs/1834479
BugLink: https://bugs.launchpad.net/bugs/1834476

SRU Justification

Impact: The nvidia-graphics-drivers-418 package landed in disco shortly
before release and was not included in linux-restricted-modules, though
it is wanted there. Also included is a fix for an existing bug observed
during testing, whereby both a signed and an unsigned module are
installed under /lib/modules, and depmod can end up preferring the
unsigned module over the signed module.

Fix: Update the linux and linux-restricted-modules packages to include
nvidia-graphics-drivers-418, and remove the unsigned module from
/lib/modules/... after generating the signed module.

Test Case: Confirm that binary packages for nvidia-418 are built and
function as expected, and confirm that nvidia-390 continue to work as
before. A test build is available in ppa:sforshee/test-builds.

Regression Potential: The nvidia-418 packages are new binary packages
which must be installed by the user, so they cannot cause regressions.
There is a risk of inadvertent errors affecting the nvidia-390 packages,
however regression testing has turned up no such regressions.

Note that the first two patches are for linux, and the final patch is
for linux-restricted-modules. These have already been applied to
unstable.

Thanks,
Seth