Make Hierarchy a configurable option

[joylatten]

I have a use case where I would like to use the NULL hierarchy with transient objects and when VM rebooted to a pristine state the objects removed and new seed generated.

[williamcroberts]

Related to #558 and #559

[williamcroberts]

I have a use case where I would like to use the NULL hierarchy with transient objects and when VM rebooted to a pristine state the objects removed and new seed generated.

So really something like tpm2_ptool init --heirarchy=null. Then on reboot of the VM just wipe the store?

Internally we use either a transient parent that gets created when the --transient-parent option or a persistent handle. We won't be able to use the persistent handle here since that only works for owner and platform hierarchies IIRC. So that --hierarchy=null would imply a transient parent.