Build system: Fixes #2: Add multi-platform support

Move all explicit platform or architecture specific references
into a new platform.mk file that is defined for each platform.

Change-Id: I9d6320d1ba957e0cc8d9b316b3578132331fa428
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
diff --git a/docs/change-log.md b/docs/change-log.md
index 7d6d8f6..50e1111 100644
--- a/docs/change-log.md
+++ b/docs/change-log.md
@@ -98,6 +98,11 @@
     system. The GICv3 re-distributors are probed to work out which
     re-distributor should be used with which CPU.
 
+*   Add multi-platform support to the build system. The user may now specify
+    which platform to build using PLAT=<platform> as part of the make command
+    line.  Default behaviour is to make all platforms.  New platforms are
+    automatically detected by the make file when they are added to the plat
+    directory.
 
 ARM Trusted Firmware - version 0.2
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