reproducibile_build: Fix SDE file generation when unpack reruns

Currently, if an existing TMPDIR is rebuilt, do_fetch/do_unpack can rerun
but SDE would remain unchanged. This leads to different results compared
to a fresh build. An example change which triggered this is:
http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=cb4e69e6346a9fbeebf83a5d5397cacbd41d48b5



Instead, delete any existing SDE and recalculate if we're reunning.
Also rename and drop the do_ prefix since these are for tasks,
not functions.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>