don't use the macports mirror, because it's not certain
whether those tarballs will always be there. use the
coreboot one as a backup instead, and nasm.us as main
Signed-off-by: Leah Rowe <leah@libreboot.org>
support redundant downloads, and enable inclusion of these
tarballs inside release archives, for offline builds.
Signed-off-by: Leah Rowe <leah@libreboot.org>