manual/user guide/customization: rework section on BR2_EXTERNAL

This patch reworks the section on BR2_EXTERNAL as follows:
- move note about upstreaming to the chapter introduction
- streamline the section with the previously added section 'Recommended
  directory structure', avoiding duplication.
- use $(BR2_EXTERNAL) rather than BR2_EXTERNAL when referring to file paths.
- some general rewording

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>