build: allow platform makefiles to configure `ENABLE_LTO`

This change introduces a lazily-evaluated condition on `ENABLE_LTO` to
the `LTO_CFLAGS` variable as opposed to evaluating the condition
eagerly.

This concludes a recent request on the mailing list:

    https://lists.trustedfirmware.org/archives/list/tf-a@lists.trustedfirmware.org/thread/EU3XR4VB3RP2NQB372QPZ4VRP57ANNLC/

Change-Id: Ie1f73352eb51fb2ceb2385232336312216ef87fc
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/Makefile b/Makefile
index 603c8d9..ef570b2 100644
--- a/Makefile
+++ b/Makefile
@@ -155,11 +155,9 @@
 	endif
 
 else ifeq ($($(ARCH)-cc-id),gnu-gcc)
-	ifeq ($(ENABLE_LTO),1)
-		# Enable LTO only for aarch64
-		ifeq (${ARCH},aarch64)
-			LTO_CFLAGS	=	-flto
-		endif
+	# Enable LTO only for aarch64
+	ifeq (${ARCH},aarch64)
+		LTO_CFLAGS	=	$(if $(filter-out 0,$(ENABLE_LTO)),-flto)
 	endif
 endif #(clang)