manual/user guide: rework 'Configuration of other components' chapter

This patch reworks the chapter 'Configuration of other components' as
follows:
- remove separate customize-xxx-config.txt files in favor of one shared file
  configure-other-componenents.txt
- use labeled list instead of separate sections, as the information in each
  section is limited.
- simplify instructions for busybox/uclibc thanks to the kconfig-package
  infrastructure
- rewrite instructions for Linux
- add instructions for Barebox

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>