MIPS: Move Qemu MIPS target to boards.cfg

CONFIG_QEMU_MIPS is already provided by <configs/qemu-mips.h>, so we
don't generate it using the options fields in boards.cfg.

Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>
diff --git a/boards.cfg b/boards.cfg
index f1f2506..cb67d2a 100644
--- a/boards.cfg
+++ b/boards.cfg
@@ -221,6 +221,7 @@
 gth2                         mips        mips
 pb1000                       mips        mips        pb1x00              -              -           pb1x00:PB1000
 purple                       mips        mips
+qemu_mips                    mips        mips        qemu-mips           -              -           qemu-mips
 tb0229                       mips        mips
 PCI5441                      nios2       nios2       pci5441             psyent
 PK1C20                       nios2       nios2       pk1c20              psyent