[U-Boot] libfdt SWIG build error with Python 3.x

[Ricardo Martins]

In some Linux distributions (e.g., Arch Linux) the Python binary points to
Python 3.x instead of Python 2.x. This is an issue when building the libfdt
SWIG extension, as the generated extension file will be called something
like _libfdt.cpython-36m-x86_64-linux-gnu.so instead of just _libfdt.so. By
simply changing python to python2 in tools/Makefile this issue goes away.
Should I submit a patch to fix this ?