toolchain/toolchain-external/toolchain-external-bootlin: finalize package addition

toolchain/toolchain-external/toolchain-external-bootlin: finalize package addition

This commit wires-up the toolchain-external-bootlin package into
Buildroot by:

 - Adding
   toolchain/toolchain-external/toolchain-external-bootlin/Config.in,
   which is not generated by the bl-toolchains-gen script as it is a
   static file that does not depend on the list and characteristics of
   available Bootlin toolchains.

 - Including that file, as well as the Config.in.options file, from
   toolchain/toolchain-external/Config.in.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Titouan Christophe <titouan.christophe@railnova.eu>
Tested-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>