efi: rng: combine bootloader provided RNG seed with RNG protocol output

Instead of blindly creating the EFI random seed configuration table iff
RNG protocol is implemented and works, check whether such a
configuration table was provided by an earlier boot stage and if so,
combine its contents with a Linux specific personalization string, and
the output of the RNG protocol (if it works).

This can be used for, e.g., systemd-boot, to pass an additional seed to
Linux in a way that can be consumed by the kernel very early.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>