ext-toolchain: Take into account Mentor Graphics acquisition of CodeSoucery

Now that CodeSourcery has been bought by Mentor Graphics, the
toolchains are named "Sourcery CodeBench". We rename the config short
description and adjust the help text, but we keep the option name in
order not to break existing configurations.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>