'source' is actually a "bashism" and the POSIX-way of sourcing a file
uses the single dot (which is arguably less readable). Both yield the
same result, and since the script may now also be sourced from
within the hacking directory, this commit expands the usage synopsis
accordingly.

Signed-off-by: martin f. krafft <madduck@madduck.net>

Conflicts:
	hacking/env-setup