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This patch extends the 'cert_create' tool to support ECDSA keys
to sign the certificates. The '--key-alg' command line option
can be used to specify the key algorithm when invoking the tool.
Available options are:

    * 'rsa': create RSA-2048 keys (default option)
    * 'ecdsa': create ECDSA-SECP256R1 keys

The TF Makefile has been updated to allow the platform to specify
the key algorithm by declaring the 'KEY_ALG' variable in the
platform makefile.

The behaviour regarding key management has changed. After applying
this patch, the tool will try first to open the keys from disk. If
one key does not exist or no key is specified, and the command line
option to create keys has been specified, new keys will be created.
Otherwise an error will be generated and the tool will exit. This
way, the user may specify certain keys while the tool will create
the remaining ones. This feature is useful for testing purposes
and CI infrastructures.

The OpenSSL directory may be specified using the build option
'OPENSSL_DIR' when building the certificate generation tool.
Default is '/usr'.

Change-Id: I98bcc2bfab28dd7179f17f1177ea7a65698df4e7