arm_kernel_bothdr.awk: Update to latest ota

The latest ota is the first one in FreeBSD that treats 0 + "0xf" as
being '0' instead of '15'. Don't use this old trick anymore to convert
from hexidecimal to a number. Write a function to do that instead. This
fixes kernel.bin building on arm*. awk on 14 doesn't need this, but to
build FreeBSD stable/14's kernel.bin on 15 we'll need it, so fast MFC.

MFC After: 3 days
Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D44801

sys: Remove $FreeBSD$: one-line sh pattern

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

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

arm64: Don't rely on host readelf for u-boot booti image generation

readelf is not a bootstrap tool and so cannot be relied upon to exist.
On macOS there is no system readelf, and even on Linux or FreeBSD where
it does exist, BUILD_WITH_STRICT_TMPPATH builds won't be able to use it.
Instead of making it a bootstrap tool, just use nm as that suffices and
already is a bootstrap tool.

Fixes: 28482babd08a ("arm64: Use new arm_kernel_boothdr script for generating booti images.")
Reviewed by: emaste, mmel
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D32734

booti: Enable loading the kernel image to any address aligned to 2 MB

We've supported this for a long time, plus most u-boot setups quietly expect
it. Otherwise they fail with different levels of memory overwrites.
MFC after: 2 weeks

sys/tools: Add a tool for generating arm and arm64 kernel images.

This tool can generate kernel images without changing the offsets in
the final executable. It replaces the ELF header by properly sized zeroed
block then emits a relative jump to _start(for 'v7jump' or 'v8jump' option)
or the booti header (for 'v8booti' option) to the beginning of the converted file.
Submited by: ian