Support turning phony warnings into errors

These still default to warnings, but if a board decides to set
BOARD_BROKEN_PHONY_TARGETS := false, they'll turn into errors. More
likely I'll just be marking the broken targets as broken, then switching
the logic here to be like the dup rules warnings/errors.

Test: On a build with warnings, try <missing>, "false", and "true"
Change-Id: I041e1ff4618c4114ec43015b6c0ae1b49b36b6f8
diff --git a/ui/build/kati.go b/ui/build/kati.go
index de8dbf4..b54872c 100644
--- a/ui/build/kati.go
+++ b/ui/build/kati.go
@@ -93,6 +93,10 @@
 		args = append(args, "--werror_overriding_commands")
 	}
 
+	if !config.BuildBrokenPhonyTargets() {
+		args = append(args, "--werror_real_to_phony", "--werror_phony_looks_real")
+	}
+
 	if !config.Environment().IsFalse("KATI_EMULATE_FIND") {
 		args = append(args, "--use_find_emulator")
 	}