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This patch extends the platform port by adding an API that returns
either the Root of Trust public key (ROTPK) or its hash. This is
usually stored in ROM or eFUSE memory. The ROTPK returned must be
encoded in DER format according to the following ASN.1 structure:

    SubjectPublicKeyInfo  ::=  SEQUENCE  {
        algorithm           AlgorithmIdentifier,
        subjectPublicKey    BIT STRING
    }

In case the platform returns a hash of the key:

    DigestInfo  ::= SEQUENCE {
        digestAlgorithm     AlgorithmIdentifier,
        keyDigest           OCTET STRING
    }

An implementation for ARM development platforms is provided in this
patch. When TBB is enabled, the ROTPK hash location must be specified
using the build option 'ARM_ROTPK_LOCATION'. Available options are:

    - 'regs' : return the ROTPK hash stored in the Trusted
      root-key storage registers.

    - 'devel_rsa' : return a ROTPK hash embedded in the BL1 and
      BL2 binaries. This hash has been obtained from the development
      RSA public key located in 'plat/arm/board/common/rotpk'.

On FVP, the number of MMU tables has been increased to map and
access the ROTPK registers.

A new file 'board_common.mk' has been added to improve code sharing
in the ARM develelopment platforms.

Change-Id: Ib25862e5507d1438da10773e62bd338da8f360bf