toolchain/external: add MIPS Codescape IMG GNU Linux toolchain

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

Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>