[PATCH v2 0/6][linux-signed*][SRU Bionic][SRU Cosmic][SRU Bionic] Correct source package in Built-Using control field

[dann frazier]

BugLink: https://bugs.launchpad.net/bugs/1824016

Backports of patches already applied to Disco.

The Built-Using field hardcodes "linux" as the source package, even for the
hwe & hwe-edge packages. Correct that. Technically only the
linux-signed-hwe{-edge} packages need this fix, but I'm including patches
for linux-signed as well to prevent future occurences due to cargo-culting.

Verified by confirming that the same control file is generated before
and after this change for linux-signed, and Built-Using is now correct
for the hwe packages.

v2:
  * Clarify the vague PACKAGE, VERSION template vars by prefixing
    them with "UNSIGNED_SRC_".