Make:Add realclean to .PHONY list

Update the cert_create Makefile to list realclean as .PHONY
(like clean)

Change-Id: I9dc8a61a11574a044372e0952b5b12b74e133747

tools/cert_create/Makefile

@@ -79,7 +79,7 @@ LIB := -lssl -lcrypto
 CC := gcc
 RM := rm -rf
 
-.PHONY: all clean
+.PHONY: all clean realclean
 
 all: clean ${BINARY}