MIPS: Refactor legacy defconfigs

Define legacy defconfigs which have been converted to the generic
platform more programatically, so that they can be listed in the
Makefile help text and as a separate Makefile target without
duplication.

Signed-off-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Matt Redfearn <matt.redfearn@mips.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kbuild@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/18596/

arch/mips/Makefile

@@ -546,14 +546,15 @@ generic_defconfig:
 # now that the boards have been converted to use the generic kernel they are
 # wrappers around the generic rules above.
 #
-.PHONY: sead3_defconfig
-sead3_defconfig:
-	$(Q)$(MAKE) -f $(srctree)/Makefile 32r2el_defconfig BOARDS=sead-3
+legacy_defconfigs		+= sead3_defconfig
+sead3_defconfig-y		:= 32r2el_defconfig BOARDS=sead-3
 
-.PHONY: sead3micro_defconfig
-sead3micro_defconfig:
-	$(Q)$(MAKE) -f $(srctree)/Makefile micro32r2el_defconfig BOARDS=sead-3
+legacy_defconfigs		+= sead3micro_defconfig
+sead3micro_defconfig-y		:= micro32r2el_defconfig BOARDS=sead-3
 
-.PHONY: xilfpga_defconfig
-xilfpga_defconfig:
-	$(Q)$(MAKE) -f $(srctree)/Makefile 32r2el_defconfig BOARDS=xilfpga
+legacy_defconfigs		+= xilfpga_defconfig
+xilfpga_defconfig-y		:= 32r2el_defconfig BOARDS=xilfpga
+
+.PHONY: $(legacy_defconfigs)
+$(legacy_defconfigs):
+	$(Q)$(MAKE) -f $(srctree)/Makefile $($@-y)