MFC r329190, r329315, r330131: metadata load file unification

r329190: Unify metadata load files for arm, mips, powerpc, sparc64

Summary:
All metadata.c files are very similar, with only trivial changes. Unify them
into a single common file, with minor special-casing where needed.

r329315: stand: Fix ubldr after r329190

metadata load files were consolidated in r329190, and these relocation fixup
bits were inadvertently dropped in the process. Re-add them to fix boot with
ubldr.

r330131: Fix module loading on arm after the metadata.c unification in
r329190. Arm modules need an additional address fixup not needed by other
platforms.

MFC r325834,r325997,326502: Move sys/boot to stand/

This is effectively a direct commit to stable/11, due to differences between
stable/11 and head. Changes to DTS in sys/boot/fdt/dts were often
accompanied by kernel changes. Many of these were also risc-v updates that
likely had many more dependencies to MFC.

Because of this, sys/boot/fdt/dts remains as-is while everything else in
sys/boot relocates to stand/.

r325834: Move sys/boot to stand. Fix all references to new location

r325997: Remove empty directories.

r326502: Document the sys/boot -> stand move in hier.7 and the top-level README.