kbuild: delete *.pyc files by "make distclean"

The tools "buildman" and "patman" are written in Python.
When we run them, "*.pyc" files are created under
tools/buildman, tools/patman directories.

They should be cleaned up by "make distclean".

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Tom Rini <trini@ti.com>
Acked-by: Simon Glass <sjg@chromium.org>