docs/manual/configure.txt: explain how to reuse a Buildroot toolchain as external...

docs/manual/configure.txt: explain how to reuse a Buildroot toolchain as external toolchain

Laurent reported that a short tutorial was missing in the manual to
explain how to reuse a Buildroot toolchain as external toolchain.

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Laurent Guillier <laurent.guillier@smile.fr>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Tested-by: Matt Weber <matthew.weber@rockwellcollins.com>
[Thomas: completely rewrite the thing]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>