MIPS: generic: Convert SEAD-3 to a generic board

Convert the MIPS SEAD-3 board support to be a generic board, supported
by generic kernels.

Because the SEAD-3 boot protocol was defined long ago and we don't want
to force a switch to the UHI protocol, SEAD-3 is added as a legacy board
which is detected by reading the REVISION register. This may technically
not be a valid memory read & future work will include attempting to
handle that gracefully. In practice since SEAD-3 is the only legacy
board supported by the generic kernel so far the read will only happen
on SEAD-3 boards, and even once Malta is converted the same REVISION
register exists there too. Other boards such as Boston, Ci20 & Ci40 will
use the UHI boot protocol & thus not run any of the legacy board detect
functions.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14354/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/Makefile b/arch/mips/Makefile
index f3e768a..fbf40d3 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -488,3 +488,16 @@
 # Prevent generic merge_config rules attempting to merge single fragments
 #
 $(generic_config_dir)/%.config: ;
+
+#
+# Legacy defconfig compatibility - these targets used to be real defconfigs but
+# 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) 32r2el_defconfig BOARDS=sead-3
+
+.PHONY: sead3micro_defconfig
+sead3micro_defconfig:
+	$(Q)$(MAKE) micro32r2el_defconfig BOARDS=sead-3