sefltest/ima: support appended signatures (modsig)

In addition to the PE/COFF and IMA xattr signatures, the kexec kernel
image can be signed with an appended signature, using the same
scripts/sign-file tool that is used to sign kernel modules.

This patch adds support for detecting a kernel image signed with an
appended signature and updates the existing test messages
appropriately.

Reviewed-by: Petr Vorel <pvorel@suse.cz>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Reviewed-by: Jordan Hand <jorhand@linux.microsoft.com> (x86_64 QEMU)
Tested-by: Jordan Hand <jorhand@linux.microsoft.com> (x86_64 QEMU)
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>