Update user guide further to linker scripts changes

This patch updates the user guide section about the memory layout.
  - Explain the verifications that the linker scripts does on the
    global memory layout.
  - Refer to the new linker symbols.
  - Describe the linker symbols exported to the trusted firmware code.

Change-Id: I033ab2b867e8b9776deb4185b9986bcb8218f286
diff --git a/docs/porting-guide.md b/docs/porting-guide.md
index 07d0f66..a3ef99e 100644
--- a/docs/porting-guide.md
+++ b/docs/porting-guide.md
@@ -164,11 +164,12 @@
 *   **#define : BL2_BASE**
 
     Defines the base address in secure RAM where BL1 loads the BL2 binary image.
+    Must be aligned on a page-size boundary.
 
 *   **#define : BL31_BASE**
 
     Defines the base address in secure RAM where BL2 loads the BL3-1 binary
-    image.
+    image. Must be aligned on a page-size boundary.
 
 
 ### Other mandatory modifications