Introduce TF_LDFLAGS

Use TF_LDFLAGS from the Makefiles, and still append LDFLAGS as well to
the compiler's invocation. This allows passing extra options from the
make command line using LDFLAGS.

Document new LDFLAGS Makefile option.

Change-Id: I88c5ac26ca12ac2b2d60a6f150ae027639991f27
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>
diff --git a/lib/cpus/cpu-ops.mk b/lib/cpus/cpu-ops.mk
index 218a548..ad32971 100644
--- a/lib/cpus/cpu-ops.mk
+++ b/lib/cpus/cpu-ops.mk
@@ -145,10 +145,10 @@
 
 # Errata build flags
 ifneq (${ERRATA_A53_843419},0)
-LDFLAGS_aarch64		+= --fix-cortex-a53-843419
+TF_LDFLAGS_aarch64	+= --fix-cortex-a53-843419
 endif
 
 ifneq (${ERRATA_A53_835769},0)
 TF_CFLAGS_aarch64	+= -mfix-cortex-a53-835769
-LDFLAGS_aarch64		+= --fix-cortex-a53-835769
+TF_LDFLAGS_aarch64	+= --fix-cortex-a53-835769
 endif