MIPS: Prevent direct use of generic_defconfig

Using generic_defconfig directly is unlikely to be what a user actually
wants to do - it doesn't specify any particular ISA revision & it
doesn't enable any board or driver support, resulting in a largely
useless kernel.

Prevent users from using it directly, printing a helpful message to
point them in the right direction if they attempt to.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16946/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Makefile

@@ -515,6 +515,19 @@ $(generic_defconfigs):
 #
 $(generic_config_dir)/%.config: ;
 
+#
+# Prevent direct use of generic_defconfig, which is intended to be used as the
+# basis of the various ISA-specific targets generated above.
+#
+.PHONY: generic_defconfig
+generic_defconfig:
+	$(Q)echo "generic_defconfig is not intended for direct use, but should instead be"
+	$(Q)echo "used via an ISA-specific target from the following list:"
+	$(Q)echo
+	$(Q)for cfg in $(generic_defconfigs); do echo "  $${cfg}"; done
+	$(Q)echo
+	$(Q)false
+
 #
 # 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