create-dev-env.sh:  if run from repo, set $BASE appropriately

2 changes:
[1] If PROJECT_HOME is not set, AND create-dev-env.sh is run from a cloned repo, then set BASE to that repo;
else:  (and only finally) set BASE to a default of "$HOME/edx_all"

[2] if PROJECT_HOME is set, or if this is from a repo, don't add edx_all to the BASE name (no need;)