When setting up the build root, only bind mount the `/boot` dir
from the supplied build tree, if the build tree is not the host
itself, since we never want to leak any host specific data and
the `/boot` directory should never be needed when building the
build root. The only reason `/boot` is mounted at all is for
the grub2 stage to copy efi binaries to the tree since they
directly installed to `/boot` by the respective bootloader
packages.