toolchain/external: add MIPS Codescape MTI GNU Linux toolchain

[Thomas:
 - rebase on top of master
 - remove version number of the Config.in option name.]

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>