[development] register the install profile in system table?

[Nedjo Rogers]

We have the ability to include _install functions in install profiles. What 
about updates? We're currently registering the install profile through 
variable_set('install_profile'). Should we instead be setting it as a row in 
the system table? That way we could track its schema version like we do for 
modules and so invoke e.g. default_update_0() when update.php is run.