Pick out the kernel binaries and add them to a custom upload.  This upload
will trigger signing of the contained files which will later be pulled
into linux-*-signed packages.

Only include amd64 kernels as we only support EFI signed packages there.
Also ensure the kernel has a high enough interface version >= 0x020b
otherwise we may end up with an unsafe kernel loaded.
Signed-off-by: Andy Whitcroft <apw@canonical.com>