mpfr: move so symlinks to -devel pkg; not a bootstrap pkg.
gmp: not a bootstrap pkg.
kernel-libc-headers: misc tweaks.
binutils: prepare to be a bootstrap pkg; supersedes chroot-binutils.
chroot-gcc: use CHROOT_READY for bootstrap pkgs.
chroot-texinfo: misc tweaks.
base-chroot: drop chroot-binutils; remove unnecessary deps.