Restore functionality to set the samples directory via the
FATE_SAMPLES environment variable . This is broken since commit
63dcd16d was merged.

  Additionally the name FATE_EXTERN is more suited as the current
FATE_SAMPLES make file variable does not carry the name of the
FATE samples or the name of the directory they are stored in, but
does contain the names of the FATE targets that need external
samples. That is samples that are not in the repository and are
not generated on the fly.
Signed-off-by: Alexander Strasser <eclipse7@gmx.net>