mkeficapsule: Add support for embedding public key in a dtb

Add options for embedding the public key esl(efi signature list) file
to the platform's dtb. The esl file is then retrieved and used for
authenticating the capsule to be used for updating firmare components
on the platform.

The esl file can now be embedded in the dtb by invoking the following
command
mkeficapsule -K <pub_key.esl> -D <dtb>

In the scenario where the esl file is to be embedded in an overlay,
this can be done through the following command
mkeficapsule -O -K <pub_key.esl> -D <dtb>

This will create a node named 'signature' in the dtb, and the esl file
will be stored as 'capsule-key'

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>