MFC r326992: embed_mfs: support embedding mfs into loader

The script originally supported embedding an mfs into ELF files or any
other type of file, because it searched for magic strings to mark the
beginning and end of the embeddable section. It was later modified to
read the section offset and length via readelf, which made it work for
ELF only. Restore the ability to update arbitrary file types by using
the readelf technique for ELF, and the magic string technique for all
others (including PE/COFF files like loader.efi).

Submitted by: Zakary Nafziger <worldofzak@gmail.com>

MFC r327526: embed_mfs: correctly test grep return value

Reported by: br

MFC r327527: ath: revert accidental change committed with r327526

Sponsored by: The FreeBSD Foundation

MFC r324707: embed_mfs: add error handling, usage

Ensure that we are called with two arguments, and that the output file
is writable. Also, if we cannot find the mfs section report the output
file name rather than "kernel", as this script may be used with other
than kernels.

Sponsored by: The FreeBSD Foundation

MFC r311881:

Replace using of objdump with elfdump

In-tree objdump is too old to dump new ELF headers. But for example if we
use: `make CROSS_TOOLCHAIN=riscv64-gcc TARGET_ARCH=riscv64` and do not specify
CROSS_BINUTILS_PREFIX in env, embed_mfs.sh cannot find the correct objdump.
This patch just replaces using of objdump with elfdump to collect needed
information.

Later we may also put an ELFDUMP in CROSSENV and use it in embed_mfs.sh .

Reviewed by: emaste, br
Differential Revision: https://reviews.freebsd.org/D9062

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Fix MFS builds when both MD_ROOT_SIZE and MFS_IMAGE are specified

MD_ROOT_SIZE and embed_mfs.sh were basically retired as part of
https://reviews.freebsd.org/D2903 .
However, when building a kernel with 'options MD_ROOT_SIZE' specified, this
results in a non-working MFS, as within sys/dev/md/md.c we fall within the
wrong # ifdef.

This patch implements the following:

* Allow kernels to be built without the MD_ROOT_SIZE option, which results
in a kernel built as per D2903.
* Allow kernels to be built with the MD_ROOT_SIZE option, which results
in a kernel built similarly to the pre-D2903 way, with the following
differences:
* The MFS is now put in a separate section within the kernel (oldmfs,
so it differs from the mfs section introduced by D2903).
* embed_mfs.sh is changed, so it looks up the oldmfs section within the
kernel, gets its size and offset, sees if the MFS will fit within the
allocated oldmfs section and only if all is well does a dd of the MFS
image into the kernel.

Submitted by: Stanislav Galabov <sgalabov@gmail.com>
Reviewed by: brooks, imp
Differential Revision: https://reviews.freebsd.org/D5093

Introduce a standalone shell script for embedding MFS image.

This allows to fix a problem with ARM kernel.bin not having the MFS image
embedded: it is objcopied from the kernel.noheader temporary ELF file, which
was not subject to embedding the MFS image previously.

Reviewed by: imp
Approved by: cognet (mentor)