target.mk: change i386 CPU type to pentium-mmx

f4f8f4a180 broke ffmpeg compilation with x86

The reason is that ffmpeg's x86 assembly requires at least MMX, which the
pentium CPU_TYPE was preventing.

Fixes ffmpeg compilation on x86_legacy and x86_geode.

Ref: https://github.com/openwrt/openwrt/pull/3061
Signed-off-by: Rosen Penev <rosenp@gmail.com>

include/target.mk

@@ -174,8 +174,8 @@ ifeq ($(DUMP),1)
     CPU_CFLAGS_octeonplus = -march=octeon+ -mabi=64
   endif
   ifeq ($(ARCH),i386)
-    CPU_TYPE ?= pentium
-    CPU_CFLAGS_pentium = -march=pentium-mmx
+    CPU_TYPE ?= pentium-mmx
+    CPU_CFLAGS_pentium-mmx = -march=pentium-mmx
     CPU_CFLAGS_pentium4 = -march=pentium4
   endif
   ifneq ($(findstring arm,$(ARCH)),)