trunks_send: Add --u2f_cert option

Add a command-line option to request and dump the individual attestation
certificate in the cr50 firmware supports the U2F feature (and its
extensions).

BUG=b:35545754
TEST='trunks_send --u2f_cert --crt=test.crt --nonce=abc --appid=dummy'
and manually check the x.509 certificate in test.crt.

Change-Id: Ibc5e3eb8665e70515c81013bd326c193634420ef
1 file changed