loader.efi: efi_exit should return more data

[tsoome]

Summary

UEFI specification has an mechanism to pass data via
EFI_BOOT_SERVICES.Exit(). We can use this to pass
panic() message, so the use has an chance to get this
information in case the console is not usable or
available.

To do this, we allocate static string buffer (arbitrary
picked 2 lines - 160 chars) - it is possible the loader heap
is corrupted and malloc is not usable, and we pass it to BS->Exit().

Note: we still need to print this buffer on console too
for non-UEFI platforms.

The static buffer size does limit the length of the panic
message (assert may generate long messages), but we do not want to
waste too much of precious resource.

[imp]

I'd really rather not have efi_main depend on the frame buffer.

stand/efi/loader/efi_main.c

49

we're panicing. Can we really allocate this? We should preallocate this maybe?

59

Is there some way we can have this be some sort of 'atexit' functionality rather than doing it inline here forcing efi_main.c to know about framebuffer stuff. It's supposed to be just a thin wrapper between stand world and efi world.

[tsoome]

preallocate buffer space for exit message.
factor out framebuffer cleanup.
we do not really return from efi_main.