[PATCH] Add support for 64bits payloads

[Thomas Jourdan]

Hi guys

This series of patches adds support for 64 bits payloads. With them, it
is possible to use a 64 bit payload. It works with old elf boot process
and new CBFS selfboot. With CBFS, when parsing the ELF, the elfclass
will be stored in a new field of the cbfs_payload structure. This field
is used later to decide or not if we need to switch to 64 bits mode
before jumping into the payload.

64 bits mode needs paging and paging needs some memory to store the page
tables. It's hardcoded and needs to be improved.

It's far from perfect and I don't think it can be included as it in the
main coreboot trunk. But it may give some ideas.

It is required so :

Signed-off-by: Thomas Jourdan

But all the assembly code to switch to 64 bits mode is taken from the
EDK2 project (https://edk2.tianocore.org). So credits are for them.

Regards,
Thomas