MFC r281169, r293724, r293796, r294029, r294041, r294058

MFC r281169 (by andrew):
Make global variabled only used in this file static

MFC r294058:
Make common boot file_loadraw name parameter const

MFC r294041:
Remove unused reg param from fdt_fixup_memory

MFC r293724:
Enable warnings in EFI boot code

MFC r293796:
Fix typo in libefi.c

MFC r294029:
Only build EFI components on supported compilers

Sponsored by: Multiplay

MFC r293233: loader.efi: add terminal emulation support

This is based on the vidconsole implementation.

Submitted by: Toomas Soome <tsoome@me.com>
Relnotes: Yes

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Reset the text attributes when initializing the console. The EFI
loader typically doesn't do this so that we end up booting the
with whatever the EFI loader has set it to last.

Use __FBSDID rather than rcsid[].

In efi_cons_poll we check if a key is present (pending) by checking
the signaled state of the apropriate event. As a side-effect of
checking the event, it's signaled state is cleared if it was set.
In efi_cons_getchar we used to wait for the apropriate event to be
signaled before reading a character. This however does not work if
we poll before reading the characteri, such as during autoboot. On
a more compliant EFI implementation this resulted in the behaviour
that hitting a key during autoboot would stop the countdown, but
would then wait for a new character to arrive instead of reading
the already pending key that stopped the countdown.

The correct behaviour for efi_cons_getchar is to try to read a key
and if none is pending, to wait for the apropriate event to signal
the arrival of a new key.

Note that with the previous behaviour, the second key would determine
how the autoboot was interrupted. This would indicate that the first
key got lost. This indicates that EFI does not necessarily maintain
a queue of pending keys. FWIW...

Approved by: re (carte blanche)
French corrected by: various people :-)

Fix FreeBSD id style.

First approximation of an ia64 EFI loader. Not functional.