vars/build.vars:{host_toolchain,invariants,native_{runtime,tools,toolchain_mingw32}}: adds dependencies.
    vars/build.vars:native_toolchain: merges native_toolchain_deps, adds dependencies.
    vars/build.vars:{bzip2,expat,gdb,python2}: moved into native_toolchain, adds dependencies.
    vars/build.vars:native_packages_deps{,_{deps,x11}}:${PKG_IN_TREE}: set.
    vars/env.vars:${DEFAULT_BUILD_VARS}: adds IN_TREE.
    vars/gcc.vars: continuously echo build status messages to pipe fd #3.
    etc/build.usage: updated.
    subr/ex_pkg_dispatch.subr: enforce in-order building given ${PKG_IN_TREE} set on group.