just reqs: Install elf2tab if necessary
We don't need libtock-rs in the top-level dir anymore, so drop that requirement, install check if we have elf2tab in our PATH and attempt to install it via cargo if necessary.
We don't need libtock-rs in the top-level dir anymore, so drop that requirement, install check if we have elf2tab in our PATH and attempt to install it via cargo if necessary.