Makefile: honor PYTHON configuration properly

On some systems `python` is `python3` (for instance, Archlinux). The
`PYTHON` variable can be used to point to `python2` to have a successful
build.

The use of `PYTHON` is currently limited in the Makefile and needs to be
extended in other places:

First, pylibfdt is required to be a Python 2 binding (binman imports
pylibfdt and is only compatible Python 2), so its setup.py needs to be
called accordingly. An alternative would be to change the libfdt
setup.py shebang to python2, but the binding is actually portable. Also,
it would break on system where there is no such thing as `python2`.

Secondly, the libfdt import checks need to be done against Python 2 as
well since the Python 2 compiled modules (in this case _libdft.so) can
not be imported from Python 3.

Note on the libfdt imports: "@if ! PYTHONPATH=tools $(PYTHON) -c 'import
libfdt'; then..." is probably simpler than the currently sub-optimal
pipe.
Reviewed-by: Jonathan Gray <jsg@jsg.id.au>

Makefile

@@ -1379,7 +1379,7 @@ $(timestamp_h): $(srctree)/Makefile FORCE
 	$(call filechk,timestamp.h)
 
 checkbinman: tools
-	@if ! ( echo 'import libfdt' | ( PYTHONPATH=tools python )); then \
+	@if ! ( echo 'import libfdt' | ( PYTHONPATH=tools $(PYTHON) )); then \
 		echo >&2; \
 		echo >&2 '*** binman needs the Python libfdt library.'; \
 		echo >&2 '*** Either install it on your system, or try:'; \

scripts/Makefile.spl

@@ -369,7 +369,7 @@ ifneq ($(cmd_files),)
 endif
 
 checkdtoc: tools
-	@if ! ( echo 'import libfdt' | ( PYTHONPATH=tools python )); then \
+	@if ! ( echo 'import libfdt' | ( PYTHONPATH=tools $(PYTHON) )); then \
 		echo '*** dtoc needs the Python libfdt library. Either '; \
 		echo '*** install it on your system, or try:'; \
 		echo '***'; \

tools/Makefile

@@ -138,7 +138,7 @@ tools/_libfdt.so: $(LIBFDT_SRCS) $(LIBFDT_SWIG)
 		CPPFLAGS="$(_hostc_flags)" OBJDIR=tools \
 		SOURCES="$(LIBFDT_SRCS) tools/libfdt.i" \
 		SWIG_OPTS="-I$(srctree)/lib/libfdt -I$(srctree)/lib" \
-		$(libfdt_tree)/pylibfdt/setup.py --quiet build_ext \
+		$(PYTHON) $(libfdt_tree)/pylibfdt/setup.py build_ext \
 			--build-lib tools
 
 ifneq ($(CONFIG_MX23)$(CONFIG_MX28),)