GNUTOOLS-14900 Fix dir structure

- Identified the role of the different directories setup in
build-gnu-toolchain.sh (added notes in the header)
- Handling execdir when the script is run from a link and as a command
  in PATH
- Documented how to configure the scripts for execution (link or from
  PATH)

Change-Id: Ice837c5d431782408a2c3c330df8c9a356081481