toolchain: merge toolchain-common.in to Config.in

toolchain: merge toolchain-common.in to Config.in

toolchain-common.in is a Config.in file with an uncommon name.
It is just included by toolchain/Config.in, and toolchain/Config.in is
not that long, so instead of renaming the file, merge it to
toolchain/Config.in.

Move the raw contents from the file to the exact location it is
currently included in order to not change the order in the menu.

Update the references in the manual as well.

Suggested-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>