ima: defer arch_ima_get_secureboot() call to IMA init time

Chester reports that it is necessary to introduce a new way to pass
the EFI secure boot status between the EFI stub and the core kernel
on ARM systems. The usual way of obtaining this information is by
checking the SecureBoot and SetupMode EFI variables, but this can
only be done after the EFI variable workqueue is created, which
occurs in a subsys_initcall(), whereas arch_ima_get_secureboot()
is called much earlier by the IMA framework.

However, the IMA framework itself is started as a late_initcall,
and the only reason the call to arch_ima_get_secureboot() occurs
so early is because it happens in the context of a __setup()
callback that parses the ima_appraise= command line parameter.

So let's refactor this code a little bit, by using a core_param()
callback to capture the command line argument, and deferring any
reasoning based on its contents to the IMA init routine.

Cc: Chester Lin <clin@suse.com>
Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Link: https://lore.kernel.org/linux-arm-kernel/20200904072905.25332-2-clin@suse.com/
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reported-by: kernel test robot <lkp@intel.com> [missing core_param()]
[zohar@linux.ibm.com: included linux/module.h]
Tested-by: Chester Lin <clin@suse.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>