Basic Makefile changes for PIE

Change-Id: I0b8ccba15024c55bb03927cdb50370913eb8010c
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
diff --git a/docs/user-guide.rst b/docs/user-guide.rst
index f4ef85d..52cb45c 100644
--- a/docs/user-guide.rst
+++ b/docs/user-guide.rst
@@ -371,6 +371,10 @@
    partitioning in EL3, however. Platform initialisation code should configure
    and use partitions in EL3 as required. This option defaults to ``0``.
 
+-  ``ENABLE_PIE``: Boolean option to enable Position Independent Executable(PIE)
+   support within generic code in TF-A. This option is currently only supported
+   in BL31. Default is 0.
+
 -  ``ENABLE_PMF``: Boolean option to enable support for optional Performance
    Measurement Framework(PMF). Default is 0.