UBUNTU: [Packaging] Set do_tools_common in common vars

UBUNTU: [Packaging] Set do_tools_common in common vars

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
13d6fbbef038fdaee35200496131e6c6c77eb4b4 ("UBUNTU: [Packaging] prevent
linux-*-tools-common from being produced from non linux packages").

Fixes: 13d6fbbef038fdaee35200496131e6c6c77eb4b4
Signed-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>