docs/manual: add section about br2-external skeleton

Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
[yann.morin.1998@free.fr:
  - alphabetical order (skeleton before toolchain)
  - leave the list of choices 'open'
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>