scons: Switch to standalone version, update to 3.1.2

* Install the standalone (scons-local) version

  Advantages:
  - The regular version of SCons requires distutils during installation;
    the standalone version does not have this requirement

  Disadvantages:
  - The scons-local package is not available from the Fossies mirror
  - The patch will need to be manually updated with each version update

* Install files to $(STAGING_DIR_HOSTPKG) instead of $(STAGING_DIR_HOST)

* Write the correct shebang into scripts instead of using the wrapper

* Update to 3.1.2

Signed-off-by: Jeffery To <jeffery.to@gmail.com>