stub: when booting a kernel foo.efi then pack foo.efi.extra.d/*.{cred,raw} up as...

stub: when booting a kernel foo.efi then pack foo.efi.extra.d/*.{cred,raw} up as synthetic initrd

This adds support for the EFI stub to look for credential files and
sysext files next to the EFI kernel image being loaded, and pack them up
in an initrd cpio image, and pass them to the kernel.

Specifically, for a kernel image foo.efi it looks for
foo.efi.extra.d/*.cred and packs these files up in an initrd, placing it
inside a directory /.extra/credentials/. It then looks for
foo.efi.extra.d/*.raw and pack these files up in an initrd, placing them
inside a directory /.extra/sysexts/. It then concatenates any other
initrd with these two initrds, so they are combined.

Or in other words auxiliary files placed next to the kernel image are
picked up automatically by the EFI stub and be made available in the
initrd in the /.extra/ directory.

What's the usecase for this? This is supposed to be useful in context of
implementing fully trusted initrds, i.e. initrds that are not built
locally on the system and unsigned/unmeasured – as we do things
currently —, but instead are built by the vendor, and measured to TPM.
The idea is that a basic initrd is always linked into the kernel EFI
image anyway. This will already be sufficient for many cases. However,
in some cases it is necessary to parameterize initrds, or to extend the
basic initrds with additional subsystems (e.g. think complex storage, or
passing server info/certificates/… to initrds). The idea is that the
parameterization is done using the "credentials" logic we already have
in systemd, with these credential files (which can optionally be
encrypted+authenticated by TPM2) being placed in the ESP next to the
kernel image. And the initrd extension via the "sysext" logic we already
have in systemd too.

Note that the files read by this code are not verified immediately, they
are copied *as-is* and placed into /.extra/ in the initrd. In a trusted
environment they need to be validated later, but before first use. For
the credentials logic this should be done via the TPM2
encryption/authentication logic. For the sysext stuff the idea is that
this is done via signed images, as implemented by #20691.