kbuild: .PHONY is not a variable, but PHONY is

.PHONY is a target, not a variable.
Signed-off-by: NUlf Magnusson <ulfalizer@gmail.com>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1717,6 +1717,6 @@ endif	# skip-makefile
 PHONY += FORCE
 FORCE:
 
-# Declare the contents of the .PHONY variable as phony.  We keep that
+# Declare the contents of the PHONY variable as phony.  We keep that
 # information in a variable so we can use it in if_changed and friends.
 .PHONY: $(PHONY)