1. Add named args to make this more clear and avoid unnessecarily
setting default values for certain values ot set others positioned
later.
2. Add support for multiple tarballs in rootfs.sh.
3. Add support for extracting multiple tarballs (name dosen't matter
anymore) in helpers.sh.
4. Add comment for 4th arg to build.
1. Adds sha256sum stage to the bash build harness.
2. Adds a third argument to build(), the checksum file name. This is
used where there is more than one checksum file, most notably
in multi-stage compilations.
3. Adds checksum files to all remaining programs.
4. Adds appropriate 3rd argument where needed (coreutils, tcc-musl,
bison).
