MFC r309412,r316109,r316132:

r309412 (by imp):

dd is currently a bootstrap tool. It really doesn't have any business
being a bootstrap tool. However, for reproducible build output,
FreeBSD added dd status=none because it was otherwise difficult to
suppress the status information, but retain any errors that might
happen. There's no real reason that dd has to be a build tool, other
than we use status=none unconditional. Remove dd from a bootstrap tool
entirely by only using status=none when available. This may also help
efforts to build the system on non-FreeBSD hosts as well.

r316109:

Don't hardcode input files for stage 1/2 bootloaders; use .ALLSRC instead

This is a better pattern to follow when creating the bootloaders and doing
the relevant space checks to make sure that the sizes aren't exceeded (and
thus, copy-pasting is a bit less error prone).

r316132:

Parameterize out 7680 (15 * 512) as BOOT2SIZE, similar to sys/boot/i386/zfsboot/...

This is being done to make it easier to change in the future--this action might be
needed sooner rather than later because of gcc 6.3.0 bailing, stating that there
is negative free space left (deficit) in the boot2 bootloader.

MFC r296769: boot/efi: Prefer nm to objdump

Both objdump and nm are equally capable of reporting undefined symbols.

This gets us a step closer to building without binutils as we have an nm
implementation from ELF Tool Chain.

Sponsored by: The FreeBSD Foundation

MFC r294068, r294265

MFC r294068:
Add EFI ZFS boot support

MFC r294265:
Fix broken DPRINTF and wire up EFI_DEBUG so -DEFI_DEBUG to make works.

Relnotes: Yes
Sponsored by: Multiplay

MFC r281060, r294060, r294291, r294493, r294284:

MFC r281060:
Remove an unnecessary space in a printf call

MFC r294060:
Modularise EFI boot loader

MFC r294291 (by andrew):
Reset the filesystem cache

MFC r294493:
Fix EFI UFS caching

MFC r294284 (by emaste):
boot1: correct typo in error message

Sponsored by: Multiplay

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 r293460:

Switch EFT boot1 to use libstand

This includes a change to the Makefile comment to correct it due to the
lack of arm and i386 support in 10.x.

Sponsored by: Multiplay

MFC r281117: Move boot1.efi to the global CLEANFILES list

It's not x86 specific.

MFC r287930: Various small cleanups to EFI loader Makefiles.

MFC r281027: Clean up more x86 only options in the efi code.

MFC r281524 by andrew@: Use MACHINE in the efi loader when it is what we mean

It may not be the same as MACHINE_CPUARCH, it just happened to be the
case the architectures this code currently supports.

Fixes build failure reported by Oliver Pinter.

MFC r280950: Move the efi loaders to be under sys/boot/efi

In HEAD this was done to support UEFI on arm64. Merging to stable/10 to
facilitate the merging of later UEFI changes.