tools: allow to override python

Not force to use python from PATH. Issue was noted when building with
Yocto, because python from the distro is always taken instead of
python-native built during Yocto process.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 09b597d..8d4e605 100644
--- a/Makefile
+++ b/Makefile
@@ -348,7 +348,7 @@
 OBJDUMP		= $(CROSS_COMPILE)objdump
 AWK		= awk
 PERL		= perl
-PYTHON		= python
+PYTHON		?= python
 DTC		= dtc
 CHECK		= sparse