The leading - makes it rather inconsistent with the majority of other
projects around the world. Use the form everyone else uses.
To solve this, properly pass prefix to inner makes as was always
intended.
Fixes: https://github.com/libopencm3/libopencm3/issues/1058
Somewhat replaces some earlier work done by hg/lg, but much more
complete, so we kept it as is, because it's bringing in even more parts
after this.
Reviewed-by: Karl Palsson <karlp@tweak.net.au>
