Correct the Makefile logic for disabling PIE

In the Makefile we use findstring to locate gcc toolchains
that have PIE enabled by default.

Unfortunately the result of findstring is compared against
an integer, 1, rather than a non-empty string; the logic to
disable PIE then doesn't get applied.

This patch fixes the flag test.

Fixes: f7ec31db

 ("Disable PIE compilation option")
Change-Id: I4cd2866974e313d6b408f9681311d78a208ab468
Signed-off-by: Steve Capper <steve.capper@arm.com>

Makefile

@@ -163,7 +163,7 @@ TF_CFLAGS		+=	$(CPPFLAGS) $(TF_CFLAGS_$(ARCH))		\
 GCC_V_OUTPUT		:=	$(shell $(CC) -v 2>&1)
 PIE_FOUND		:=	$(findstring --enable-default-pie,${GCC_V_OUTPUT})
 
-ifeq ($(PIE_FOUND),1)
+ifneq ($(PIE_FOUND),)
 TF_CFLAGS		+=	-fno-PIE
 endif