toolchain-external/CodeSourcery MIPS: available only for R2

Currently the CodeSourcery toolchains for MIPS can be selected to build
mips32 (revision level 1) targets, but the resulting binaries are built
for mips32r2 instead. This is because these toolchains don't have
library support other than mips32r2, so there is no point to allow the
selection of a mips32 variant with a CodeSourcery MIPS toolchain, since
everything will be built for mips32r2 instead.

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