kboot: Centralize ldscript addition

Make the pattern for ldscripts always be
arch/$MACHINE_ARCH/$MACHINE_ARCH.ldscript so we can add it from a
central Makefile. This also moves from ldscript.arch to arch.ldscript to
match the loader's new convention.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D44057

kboot: Add our own lua bindings

Create a small wrapper around the new flua hash module so we can use it
here too. There's no 4th bindings, nor will they be created.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D43874

loader: Remove gfx_fb_stub.c, it's no longer needed

Now that we draw in the gfx bindings for all our interpreters only when
graphics support is compiled in, we can eliminate this from all the
loaders that don't have graphics support.

Sponsored by: Netflix
Reviewed by: kevans, jhb
Differential Revision: https://reviews.freebsd.org/D43905

kboot: Move termios to libkboot

Sponsored by: Netflix

kboot: Move system calls to libkboot

Sponsored by: Netflix

kboot: Cleanup libkern reference

For aarch64 and amd64, we don't pull in anything from libkern, so we
don't need it in our path. However, powerpc needs ucmpdi2 from libkern,
so bring it into libkboot's build and omit it from loader.kboot.

Sponsored by: Netflix

kboot: Move _start out of kboot and into libkboot

Move the startup code from kboot/kboot to kboot/libkboot and add the
necessary infrastructure for it to build. move start_arch.h, a private
header for libkboot, over as well.

Sponsored by: Netflix

kboot: Move to kboot/kboot

In anticipation of separating the library elements from the kboot
prorgam elements, move kboot down a level into a subdirectory. There
will be libkboot and include directories in subsequent commits,
mirroring other subsystems like i386 and efi.

Sponsored by: Netflix