tpm2_load: print usage if required options aren't specified
tpm2_load: print usage if required options aren't specified

tpm2_load requires all of -C/--context-parent, -u/--pubfile and
-r/--privfile be specified. If all options aren't passed, print the usage
information.

Signed-off-by: Joshua Lock <9750fc34162235efa862f2505c1a01f735e97af7@intel.com>

martinezjavier/tpm2-tools,01org/tpm2.0-tools,martinezjavier/tpm2-tools,01org/tpm2.0-tools,01org/tpm2.0-tools,martinezjavier/tpm2-tools,martinezjavier/tpm2-tools