build.sh: set ${PKG_NAME_PARENT} for `flavoured' packages.
    subr/build.subr: fix bug in log_env_vars().
    subr/pkg_setup_env.subr: inherit and log variables from parent packages when processing `flavoured' packages.
    vars/build.vars: replaces minipix packages w/ {bash,coreutils}_flavour_minipix, which are statically linked and installed to ${PREFIX_MINIPIX}.
    vars/musl.vars: don't install libc.so to ${PREFIX_MINIPIX}/lib.