boot/efi: implement ring buffer

Change-Id: I7c384eb859e28ce824bd5cc5e85039d80462eb67
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5722
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

EFI: Hand off bootloader log to the kernel.

Eventually, EFI should get support for the debug log ring_buffer
like bios_ia32 has for transparent handoffs, but this preserves
everything except for the pre-start MMU dump, which is an improvement.

Fixes #15455.

Bootloader log: check for buffer overflow

We could overflow the in-memory log. The bounds check was there for BIOS
already but was missing in EFI and openfirmware. Could fix some crashes
when there is lots of loging.

efi loader: implement simple in-memory log

This lets us see the bootloader log from the boot menu, which is very helpful
on machines with no serial port or other way to debug.

UEFI: serial debugging support.

Signed-off-by: Jessica Hamilton <jessica.l.hamilton@gmail.com>

EFI: stub functions so we can call loader's main() function.