BugLink: http://bugs.launchpad.net/bugs/1690908

The current module signing exclusion implementation suffers from
two problems. First, it looks for the signed-inclusion file
relative to the path where make is executed and thus doesn't work
if the source and build directories are different. Second, the
signed-inclusion file lists only the module name, but the strings
searched for in the file include the path (and the path to the
module install location at that).

Fix these problems by updating scripts/Makefile.modinst to look
for signed-inclusion relative to the path of the source tree and
to use only the module name when matching against the contents of
that file.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>