Make systemd-boot compliant with the Linux Boot / EFI Handover Protocol

Make systemd-boot compliant with the Linux Boot / EFI Handover Protocol

The current implementation copied the *complete* header to boot_params,
thus making the kernel ignore many of the fields.

As mentioned in the code comment for the sentinel variable in
bootparam.h a bootloader should only copy the setup_header, set some
fields in boot_params and zero out anything else.

This change makes systemd-boot (mostly) compliant with the Linux Boot
Protocol and the EFI Handover Protocol described in bootparam.h and
Documentation/boot.txt to fix various issues:

- Secure boot not being detected corretly by Linux (#11717)
- tboot error message / warning on boot (#11717)
- Strange purple text color when booting in qemu with OVMF
- Hopefully even more ...