GBE: define python interpreter by cmake variable
GBE: define python interpreter by cmake variable

In some distros, python is linked to python3 not
python2, and GBE can't be built on such distros
without modification.

CMake provides a variable PYTHON_EXECUTABLE.
By default, this variable is the same as
`/usr/bin/env python`, and if another python2
interpreter is needed, just add this defination in
`cmake` command.

-DPYTHON_EXECUTABLE:FILEPATH=/path/to/python2

And this will change PYTHON_EXECUTABLE to
/path/to/python2

Signed-off-by: Boqun Feng <4e4905b5cb0d447358c6a2b2a203fe3f2ade002e@gmail.com>
Reviewed-by: Zhigang Gong <e04a7b9b70b1e4c6318cf117dcd1a9056e14b97a@linux.intel.com>

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