Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

stand: Move eficom to libefi

Rename efiserialc to eficom.c and move it to libefi. Remove
loader.efi.h, since it's not needed. It's architecture independent
(though how we use it might vary). Drivers also belong in libfoo
in the boot loader: all the BIOS drivers are in i386/libi386 and
the console driver is in efi/libefi.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D40007

amd64 loader: Use efiserialio for Hyper-V booted systems

UEFI provides ConIn/ConOut handles for consoles that it supports,
which include the text-video and serial ports. When the serial port
is available, use the UEFI driver instead of direct io-port accesses
to avoid conflicts between the firmware and direct hardware access, as
happens on Hyper-V (Azure) setups.

This change enables efiserialio to be built for efi-amd64 and has
higher order priority vs comconsole, and only uses efiserialio
if the hypervisor is Hyper-V. When efiserialio successfully
probes, it will set efi_comconsole_avail=true which will prevent
comconsole from probing in this setup.

Tested on Hyper-V, ESXi and Azure VMs.

PR: 264267
Reviewed by: kevans, whu
Tested by: whu
Obtained from: Rubicon Communications, LLC (Netgate)
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC (Netgate)

stand/multiboot2: add support for booting a Xen dom0 in UEFI mode

Add some basic multiboot2 infrastructure to the EFI loader in order to
be capable of booting a FreeBSD/Xen dom0 when booted from UEFI.

Only a very limited subset of the multiboot2 protocol is implemented
in order to support enough to boot into Xen, the implementation
doesn't intend to be a full multiboot2 capable implementation.

Such multiboot2 functionality is hooked up into the amd64 EFI loader,
which is the only architecture that supports Xen dom0 on FreeBSD.

The options to boot a FreeBSD/Xen dom0 system are exactly the same as
on BIOS, and requires setting the xen_kernel and xen_cmdline options
in loader.conf.

Sponsored by: Citrix Systems R&D
Reviewed by: tsoome, imp
Differential revision: https://reviews.freebsd.org/D28497

loader: use teken teminal emulator for x86 and uefi

Replace mini cons25 emulator with teken, this does enable us proper console
terminal for loader and will make it possible to implement different
back end callbacks to draw to screen.

At this time we still only "draw" in text mode.

Move sys/boot to stand. Fix all references to new location

Sponsored by: Netflix