Doc: Minor documentation fixes

This patch introduces a set of minor bug-fixes,
aimed at increasing the quality of the rendered
documentation

It also adds a new feature of exposing cmake
variables to the documentation.

* Exposed @SPHINXCFG_TFM_VERSION@ to .rst documents
  as |TFM_VERSION|
* Upgraded tfm_its_512_flash document to comply with
  the documentation guidelines
* Fixed a typo in tfm_build_instruction document
* Reworded the cryptocell-312-runtime readme file with
  valid links to the user-manual
* Added psoc64/security/keys/readme.rst to excluded
  document-list
* Fixed an external link referencing on readme.rst

Change-Id: Ic6406fb9b59ff25f964506675a24334beeaaa738
Signed-off-by: Galanakis, Minos <minos.galanakis@arm.com>
diff --git a/docs/user_guides/tfm_build_instruction.rst b/docs/user_guides/tfm_build_instruction.rst
index b7ab2c0..7257ec7 100644
--- a/docs/user_guides/tfm_build_instruction.rst
+++ b/docs/user_guides/tfm_build_instruction.rst
@@ -214,7 +214,7 @@
             ``-DTARGET_PLATFORM=AN524``
           - Cypress CYS0644ABZI-S2D44 board (PSoC64 platform)
             ``-DTARGET_PLATFORM=psoc64``
-            See :doc:`Cypress PSoC 64 platform specifics </platform/ext/target/cypress/psoc64/cypress_psoc_64_spec>`
+            See :doc:`Cypress PSoC 64 platform specifics </platform/ext/target/cypress/psoc64/cypress_psoc64_spec>`
           - DesignStart FPGA on Cloud: Cortex-M33 based platform (SSE-200_AWS platform)
             ``-DTARGET_PLATFORM=SSE-200_AWS``
 
@@ -250,4 +250,4 @@
 
 --------------
 
-*Copyright (c) 2017-2019, Arm Limited. All rights reserved.*
+*Copyright (c) 2017-2020, Arm Limited. All rights reserved.*