docs/manual: add details on toolchain configuration

This commit adds a new section

 Working with Buildroot
  -> Details in configuration
    -> Cross-compilation toolchain

that gives a lot of details about our three toolchain backends. It
takes some elements that were previously later in the manual in
"Embedded basics" and in "External toolchain" and combines them with
more details about internal backend and Crosstool-NG backend.

The "Embedded basics" section becomes empty and is therefore
removed. A few elements have been lost in the process, but they were
considered to not be really useful, especially located so far in the
manual.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>