cpu/mips/cofigl.mk: Make a needlessly deffered expansion immediate.

This reduces the build time by ~10%. Here's the gth2_config example.

        BEFORE       AFTER
real    0m31.441s    0m27.833s
user    0m24.766s    0m23.045s
sys     0m10.425s    0m7.468s

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>