TBBR: Reduce size of ECDSA key buffers
TBBR: Reduce size of ECDSA key buffers

The TBBR implementation extracts public keys from certificates and
stores them in static buffers. DER-encoded ECDSA keys are only 91 bytes
each but were each allocated 294 bytes instead. Reducing the size of
these buffers saves 609 bytes of BSS in BL2 (294 - 91 = 203 bytes for
each of the 3 key buffers in use).

Also add a comment claryfing that key buffers are tailored on RSA key
sizes when both ECDSA and RSA keys are used.

Change-Id: Iad332856e7af1f9814418d012fba3e1e9399f72a
Signed-off-by: Sandrine Bailleux <4b69fcfca8da45b0c11ecda5434c5da526575d9d@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware