git.sh: support downloading *files* as submodules

when we download coreboot, we currently don't have a way to
download crossgcc tarballs, so we rely on coreboot to do it,
which means running the coreboot build system to do it; which
means we don't get them in release archives, unless we add
very hacky logic (which did exist and was removed).

the problem with coreboot's build system is that it does not
define backup links for each given tarball, instead relying
on gnu.org exclusively, which seems OK at first because the
gnu.org links actually return an HTTP 302 response leading
to a random mirror, HOWEVER:

the gnu.org 302 redirect often fails, and the download fails,
causing an error. a mitigation for this has been to patch the
coreboot build system to download directly from a single mirror
that is reliable (in our case mirrorservice.org).

while this mitigation mostly works, it's not redundant; the
kent mirror is occasionally down too, and again we still have
the problem of not being able to cleanly provide crossgcc
tarballs inside release archives.

do it in config/submodules, like so:

module.list shall say the relative path of a given file,
once downloaded, relative to the given source tree.

module.cfg shall be re-used, in the same way as for git
submodules, but:

subfile="url"
subfile_bkup="backup url"

do this, instead of:

subrepo="url"
subrepo_bkup="backup url"

example entries in module.list:

util/crossgcc/tarballs/binutils-2.41.tar.xz
util/crossgcc/tarballs/gcc-13.2.0.tar.xz
util/crossgcc/tarballs/gmp-6.3.0.tar.xz
util/crossgcc/tarballs/mpc-1.3.1.tar.gz
util/crossgcc/tarballs/mpfr-4.2.1.tar.xz
util/crossgcc/tarballs/nasm-2.16.01.tar.bz2
util/crossgcc/tarballs/R06_28_23.tar.gz

the "subrev" variable (in module.cfg) has been renamed
to "subhash", so that this makes sense, and that name is
common to both subfile/subrepo.

the download logic from the vendor scripts has been re-used
for this purpose, and it verifies files using sha512sum.
therefore:

when specifying subrepo(git submodule), subhash will still
be a sha1 checksum, but:

when specifying subfile(file, e.g. tarball), subhash will
be a sha512 checksum

the logic for both (subrepo and subfile) is unified, and
has this rule:

subrepo* and subfile* must never *both* be declared.

the actual configuration of coreboot crossgcc tarballs
will be done in a follow-up commit. this commit simply
modifies the code to accomodate this.

over time, this feature could be used for many other files
within source trees, and could perhaps be expanded to allow
extracting source tarballs in leiu of git repositories, but
the latter is not yet required and thus not implemented.

Signed-off-by: Leah Rowe <leah@libreboot.org>

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