Makefile: Fix verbose builds on Windows

Commit <ee1ba6d4ddf1> ("Makefile: Support totally quiet output with -s")
broke verbose (V=1) builds on Windows. This patch fixes it by adding
helpers to silence echo prints in a OS-dependent way.

Change-Id: I24669150457516e9fb34fa32fa103398efe8082d
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/Makefile b/Makefile
index 8cba50c..be543fa 100644
--- a/Makefile
+++ b/Makefile
@@ -78,12 +78,12 @@
         CHECKCODE_ARGS	+=	--no-summary --terse
 else
         Q:=
-        ECHO:=@\#
+        ECHO:=$(ECHO_QUIET)
 endif
 
 ifneq ($(findstring s,$(filter-out --%,$(MAKEFLAGS))),)
         Q:=@
-        ECHO:=@\#
+        ECHO:=$(ECHO_QUIET)
 endif
 
 export Q ECHO