Makefile: allow to override python3

Python3 taken from the PATH causes build issues when pylibfdt bindings are
generated with Yocto SDK.

Python3 provided as a part of SDK is not compatible with host Python3,
therefore binding build breaks with following errors:

scripts/dtc/pylibfdt/libfdt_wrap.c:154:11: fatal error: Python.h: No such file or directory
  154 | # include <Python.h>
      |           ^~~~~~~~~~

Do not enforce the python3 from the PATH and make it conditionally-assigned
so it can be overridden from outside of build system. Keep the default
assignment to point to version that is taken from the PATH.

Similar fix has been introduced in b48bfc74 ("tools: allow to override
python"), where conditional assignment is used for python executable to
address similar build errors.

Signed-off-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Cc: Simon Glass <sjg@chromium.org>
Fixes: e91610da ("kconfig: re-sync with Linux 4.17-rc4")
Reviewed-by: Simon Glass <sjg@chromium.org>