kbuild: allow to override Python command name

The specification of Python 3 is largely different from that of
Python 2.

For example, arch/ia64/scripts/unwcheck.py seems to be written
in Python 2, not compatible with Python 3.

It is not a good idea to invoke python scripts with the hard-coded
command name 'python'. The command 'python' could possibly be
Python 3 on some systems.
For that case, it is reasonable to allow to override the command name
by giving 'PYTHON=python2' from the command line.

The 'python' in arch/ia64/Makefile should be replaced with '$(PYTHON)'.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: linux-ia64@vger.kernel.org
Signed-off-by: Michal Marek <mmarek@suse.cz>