manual: board support: add more of our expectations

manual: board support: add more of our expectations

The manual has a section on adding board support to upstream buildroot,
but it fails to mention some of the things we expect. Add more of them.

- Internal toolchain.
- Beautify defconfig file.
- Fixed versions for components.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Adam Duskett <Aduskett@gmail.com>
Reviewed-by: Adam Duskett <aduskett@gmail.com>
[yann.morin.1998@free.fr:
  - use +monospace+ for the variables
  - use _italic_ for sections in defconfig
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>