UBUNTU: [Packaging] Fix module signing with older modinfo

UBUNTU: [Packaging] Fix module signing with older modinfo

BugLink: https://bugs.launchpad.net/bugs/1852581
Not all versions of modinfo support the signer field;
specifically, the version in boinic does not. This leaves all
modules unsigned in hwe kernels based on eoan and later. Change
the check to look for the magic string at the end of the module,
which does not rely on any external tools being aware of module
signatures.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>