pylibfdt: Use Python2 explicitly

The pylibfdt code is written for Python2, not Python3.  So, it's safer to
explicitly request Python2 in our scripts and when checking pkg-config.

On Arch Linux at least, there isn't actually a plain "python" link, just
"python2" and "python3", so the current setup won't work at all.

According to https://www.python.org/dev/peps/pep-0394/ using "python2"
should work, and is preferred.

Updating pylibfdt to work with Python3 would be nice, but is a problem for
another day.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Makefile

@@ -129,7 +129,7 @@ all: $(BIN) libfdt
 # We need both Python and swig to build pylibfdt.
 .PHONY: maybe_pylibfdt
 maybe_pylibfdt: FORCE
-	if $(PKG_CONFIG) --cflags python >/dev/null 2>&1; then \
+	if $(PKG_CONFIG) --cflags python2 >/dev/null 2>&1; then \
 		if which swig >/dev/null 2>&1; then \
 			can_build=yes; \
 		fi; \

pylibfdt/setup.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 
 """
 setup.py file for SWIG libfdt