Remove /sys/boot from legacy-build.
Add btxld to build-tools.
In src/sys/Makefile:
Add /sys/boot for i386 ELF.
I'm still not sure why the new boot code was being built along with the
legacy stuff, which meant a completely wrong default environment for it.
This may well still be the wrong way to go about this, but it can't work
all that much worse than it has been.