Add config check in Makefile to only build the subdir of current platform.

E.g. without this patch:

  AR      arch/mips/built-in.a
  AR      arch/mips/boot/dts/brcm/built-in.a
  AR      arch/mips/boot/dts/cavium-octeon/built-in.a
  AR      arch/mips/boot/dts/img/built-in.a
  AR      arch/mips/boot/dts/ingenic/built-in.a
  AR      arch/mips/boot/dts/lantiq/built-in.a
  DTC     arch/mips/boot/dts/loongson/loongson3_4core_rs780e.dtb
  DTB     arch/mips/boot/dts/loongson/loongson3_4core_rs780e.dtb.S
  AS      arch/mips/boot/dts/loongson/loongson3_4core_rs780e.dtb.o
  DTC     arch/mips/boot/dts/loongson/loongson3_8core_rs780e.dtb
  DTB     arch/mips/boot/dts/loongson/loongson3_8core_rs780e.dtb.S
  AS      arch/mips/boot/dts/loongson/loongson3_8core_rs780e.dtb.o
  AR      arch/mips/boot/dts/loongson/built-in.a
  AR      arch/mips/boot/dts/mscc/built-in.a
  AR      arch/mips/boot/dts/mti/built-in.a
  AR      arch/mips/boot/dts/netlogic/built-in.a
  AR      arch/mips/boot/dts/ni/built-in.a
  AR      arch/mips/boot/dts/pic32/built-in.a
  AR      arch/mips/boot/dts/qca/built-in.a
  AR      arch/mips/boot/dts/ralink/built-in.a
  AR      arch/mips/boot/dts/xilfpga/built-in.a
  AR      arch/mips/boot/dts/built-in.a

With this patch:

  AR      arch/mips/built-in.a
  DTC     arch/mips/boot/dts/loongson/loongson3_4core_rs780e.dtb
  DTB     arch/mips/boot/dts/loongson/loongson3_4core_rs780e.dtb.S
  AS      arch/mips/boot/dts/loongson/loongson3_4core_rs780e.dtb.o
  DTC     arch/mips/boot/dts/loongson/loongson3_8core_rs780e.dtb
  DTB     arch/mips/boot/dts/loongson/loongson3_8core_rs780e.dtb.S
  AS      arch/mips/boot/dts/loongson/loongson3_8core_rs780e.dtb.o
  AR      arch/mips/boot/dts/loongson/built-in.a
  AR      arch/mips/boot/dts/built-in.a
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>