sandbox: capsule: Add keys and certificates needed for capsule update testing

Add the private keys and public key certificates which are to be used
for capsule authentication while testing the EFI capsule update
functionality. There are two pairs of private and public keys, good
and bad. The good key pair will be used for signing capsules, whilst
the bad key pair is to be used as malicious keys for testing
authentication failure cases. The capsule_pub_key_good.crt is also
converted to an EFI Signature List(ESL) file, SIGNER.esl, which is
embedded in the platform's device-tree for capsule authentication.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>