build: cleanup possibly dangling Python 2 host symlink

build: cleanup possibly dangling Python 2 host symlink

When bumping buildroot to Python 3, we need to assure, that Python
symlink in staging bin directory points to Python >= 3.5 as well.

We can't rely completly just on SetupHostCommand as its executed only in
cases when the $(STAGING_DIR_HOST)/bin/python doesn't already exist, so
we need to remove it before running SetupHostCommand.

Acked-by: Yousong Zhou <yszhou4tech@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>