sparc: introduce arch/sparc/Kbuild

This allows us to do:

    make arch/sparc/

to build the core part of the sparc kernel.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/sparc/Kbuild

+#
+# core part of the sparc kernel
+#
+
+obj-y += kernel/
+obj-y += mm/
+obj-y += math-emu/

arch/sparc/Makefile

@@ -52,8 +52,8 @@ endif
 head-y                 := arch/sparc/kernel/head_$(BITS).o
 head-y                 += arch/sparc/kernel/init_task.o
 
-core-y                 += arch/sparc/kernel/
-core-y                 += arch/sparc/mm/ arch/sparc/math-emu/
+# See arch/sparc/Kbuild for the core part of the kernel
+core-y                 += arch/sparc/
 
 libs-y                 += arch/sparc/prom/
 libs-y                 += arch/sparc/lib/