vars/build.vars:{host,native}_toolchain: merges {host,native}_toolchain_mingw32.
    vars/build.vars:native_toolchain: fix group variables.
    vars/build.vars:ncurses: moved into native_packags_deps_deps.
    vars/build.vars:{bzip2,expat,lib{elf,ffi,readline,z}}: moved into native_packages_deps.
    vars/build.vars:{gdb,python2}: moved into native_packages.
    vars/gcc.vars: minor cleanup.
    etc/build.usage: updated.
    subr/ex_pkg_dispatch.subr: disable dependency expansion given -r.