[PATCH][SRU ALL] Update use of BACKPORT_SUFFIX to support forward ports

[Seth Forshee]

BugLink: https://bugs.launchpad.net/bugs/1902957

Our kernel packaging currently has support for backporting kernels to
older releases. This includes appending ~BACKPORT_SUFFIX to the version
string for the backport kernel, where BACKPORT_SUFFIX is defined in
update.conf. We need to start doing similar forward ported kernels, but
BACKPORT_SUFFIX should be appended using a + rather than a ~. Our kernel
packaging cannot easily determine whether a + or a ~ is appropriate.

It's much easier to determine the appropriate leader when generating
update.conf, as the script which does this has all of the information
from kernel-series at its disposal. Therefore the following patches
modify generate-update-conf to include the leader in BACKPORT_SUFFIX,
and change the kernel packaging to no longer insert a leader.

Since kteam-tools and our kernel packaging will need to be updated in
lock step, I am sending the patches as a combined series. The patch for
the kernel packaging should be applied to all supported kernels, or at
minimum all supported kernels which are a base for backport kernels.

Thanks,
Seth