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>

cpu/mips/config.mk

@@ -21,7 +21,7 @@
 # MA 02111-1307 USA
 #
 v=$(shell $(AS) --version |grep "GNU assembler" |cut -d. -f2)
-MIPSFLAGS=$(shell \
+MIPSFLAGS:=$(shell \
 if [ "$v" -lt "14" ]; then \
 	echo "-mcpu=4kc"; \
 else \