x86: Cleanup Makefile.x86_common

This patch does several cleanups in Makefile.x86_common file. The
changes are described above.

1) The CFLAGNO variable was removed since it is used only to assign
   the CFLAGS variable. Also, CFLAGNO is not used outside Makefile.x86_
   common.

2) The "-I/usr/local/include" option was removed since we provide manually
   the include path from newlib in the bsp/ directory.

3) We only support building x86-based platforms on Linux so there is no
   point in setting LDFLAGS conditionally.

4) The '-export-dynamic' option was removed from LDFLAGS since we are not
    creating a dynamically linked executable.

5) Makefile.x86_quarkX1000 is the only one that includes Makefile.x86_
   common. Since it doesn't use the custom rules from Makefile.x86_
   common we remove them.

cpu/x86/Makefile.x86_common

@@ -9,25 +9,5 @@ OBJCOPY  = objcopy
 SIZE     = size
 STRIP    = strip
 
-CFLAGSNO = -Wall -g -I/usr/local/include
-CFLAGS  += $(CFLAGSNO)
-ifeq ($(HOST_OS),Linux)
-  LDFLAGS  = -Wl,-Map=contiki-$(TARGET).map,-export-dynamic,--build-id=none
-else
-  LDFLAGS  = -Wl
-endif
-
-### Compilation rules
-
-%.so: $(OBJECTDIR)/%.o
-	$(LD) -shared -o $@ $^
-
-ifdef CORE
-.PHONY: symbols.c symbols.h
-symbols.c symbols.h:
-	$(NM) $(CORE) | awk -f $(CONTIKI)/tools/mknmlist > symbols.c
-else
-symbols.c symbols.h:
-	cp ${CONTIKI}/tools/empty-symbols.c symbols.c
-	cp ${CONTIKI}/tools/empty-symbols.h symbols.h
-endif
+CFLAGS  += -Wall -g
+LDFLAGS += -Wl,-Map=contiki-$(TARGET).map,--build-id=none