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According to the documentation [1], KEY_SIZE defaults to 2048 when RSA
algorithm is chosen, so set this value on the make's defaults file.

[1] https://trustedfirmware-a.readthedocs.io/en/latest/getting_started/build-options.html



Change-Id: I030f98363198a752bc0dd03528f748de527d48d8
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>