Adds option to read ROTPK from registers for FVP

Enables usage of ARM_ROTPK_LOCATION=regs for FVP board.
Removes hard-coded developer keys. Instead, setting
ARM_ROTPK_LOCATION=devel_* takes keys from default directory.
In case of ROT_KEY specified - generates a new hash and replaces the
original.

Note: Juno board was tested by original feature author and was not tested
for this patch since we don't have access to the private key. Juno
implementation was moved to board-specific file without changing
functionality. It is not known whether byte-swapping is still needed
for this platform.

Change-Id: I0fdbaca0415cdcd78f3a388551c2e478c01ed986
Signed-off-by: Max Shvetsov <maksims.svecovs@arm.com>
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 2f44fe8..13d7058 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -468,7 +468,8 @@
    entrypoint) or 1 (CPU reset to SP_MIN entrypoint). The default value is 0.
 
 -  ``ROT_KEY``: This option is used when ``GENERATE_COT=1``. It specifies the
-   file that contains the ROT private key in PEM format. If ``SAVE_KEYS=1``, this
+   file that contains the ROT private key in PEM format and enforces public key
+   hash generation. If ``SAVE_KEYS=1``, this
    file name will be used to save the key.
 
 -  ``SAVE_KEYS``: This option is used when ``GENERATE_COT=1``. It tells the
@@ -657,4 +658,4 @@
 
 --------------
 
-*Copyright (c) 2019, Arm Limited. All rights reserved.*
+*Copyright (c) 2019-2020, Arm Limited. All rights reserved.*