The current MIPS CPU config.mk code always expects a MIPS 4kc
core. This is not appropiate for other CPUs and SoCs.

Replace the current MIPSFLAGS code by cc-option macro and use
-march=mips32r2 as default optimization level for all MIPS32 CPUs.

Note: Since commit f62fb999 all
toolchains with binutils prior to v2.16 are not working anymore.
As agreed with Shinya Kuribayashi the support for those toolchains
will be dropped officially with this patch.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Cc: Wolfgang Denk <wd@denx.de>
Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>