kbuild: add PYTHON2 and PYTHON3 variables

The variable 'PYTHON' allows users to specify a proper executable
name in case the default 'python' does not work.  However, this does
not address the case where both Python 2.x and 3.x scripts are used
in one source tree.

PEP 394 (https://www.python.org/dev/peps/pep-0394/

) provides a
convention for Python scripts portability.  Here is a quotation:

  In order to tolerate differences across platforms, all new code
  that needs to invoke the Python interpreter should not specify
  'python', but rather should specify either 'python2' or 'python3'.
  This distinction should be made in shebangs, when invoking from a
  shell script, when invoking via the system() call, or when invoking
  in any other context.
  One exception to this is scripts that are deliberately written to
  be source compatible with both Python 2.x and 3.x. Such scripts may
  continue to use python on their shebang line without affecting their
  portability.

To meet this requirement, this commit adds new variables 'PYTHON2'
and 'PYTHON3'.

arch/ia64/scripts/unwcheck.py is the only script that has ever used
$(PYTHON).  Recent commit bd5edbe6 ("ia64: convert unwcheck.py to
python3") converted it to be compatible with both Python 2.x and 3.x,
so this is the exceptional case where the use of 'python' is allowed.
So, I did not touch arch/ia64/Makefile.

tools/perf/Makefile.config sets PYTHON and PYTHON2 by itself, so it
is not affected by this commit.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>