Toaster depends on pokydirname for identifying the location of the oe-init-build-env script and there might be other purposes in the future. 

The problem with current approach is that it only checks/sets the variable with git based repos whereas toaster provides mechanism to allow having layers that are all locally available the evaluation of the variable fails. In such scenarios so we use a more flexible discovery mechanism.

Additionally, we need to add the ability to set a custom-only fixture installation with an environment variable, not just with patching "settings.xml".

Proposal:
  * Allow a toaster command line option to set the location of "TOASTERDIR"

  * Try to also locate poky in the local layers if its path is not already discovered or set. 

  * Allow "CUSTOM_XML_ONLY" to be passed as an environment variable.