build.py: add clean option to 'devtool build' command
Add -c (--clean) optiont to 'devtool build' command so that users
could easily clean things up when using devtool.
I encountered a problem about do_prepare_recipe_sysroot failure
when using `devtool build' command and I found myself in a situation
where I either have to use `bitbake' command to clean things up or
use `rm' to remove the directories under ${WORKDIR}.
So add a clean option as it would be helpful when users want to clean
things up to prepare an environment for a clean build.
Signed-off-by: Chen Qi <Qi.Chen@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
