buildroot

docs/manual: add appendix about migration from older versions

... and move the br2-external migration to it.

Reported-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>