Move modules under /usr in signed kernel packages

For the trixie release, all packages should be installing to/usr/lib rather than lib. I already implemented this for unsigned kernel packages, but missed signed kernel packages, and those turn out to be a little more difficult.

We currently build linux-image packages first and then use each package directory as a source directory for kernel-wedge to produce installer udeb packages. But kernel-wedge still expects modules to be under /lib/modules not /usr/lib/modules.

This introduces an intermediate installation directory to the linux-signed-* source packages, mirroring the build process of the linux source package more closely. This is then installed separately into the linux-image and udeb packages.