BugLink: https://bugs.launchpad.net/bugs/1691814

In order to allow derivatives to really override do_tools_common inside
hooks.mk, it needs to be unconditionally set to true in
0-common-vars.mk, which is included before hooks.mk. Otherwise, hooks.mk
won't be able to override it, and it will be true unless other
conditions apply.

This has caused derivatives to fail to build after commit
13d6fbbe ("UBUNTU: [Packaging] prevent
linux-*-tools-common from being produced from non linux packages").

Fixes: 13d6fbbeSigned-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Andy Whitcroft <andy.whitcroft@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>