efi: Allow to enable EFI runtime services by default on RT

efi: Allow to enable EFI runtime services by default on RT

Commit d9f283ae71af ("efi: Disable runtime services on RT") disabled EFI
runtime services by default when the CONFIG_PREEMPT_RT option is enabled.

The rationale for that commit is that some EFI calls could take too much
time, leading to large latencies which is an issue for Real-Time kernels.

But a side effect of that change was that now is not possible anymore to
enable the EFI runtime services by default when CONFIG_PREEMPT_RT is set,
without passing an efi=runtime command line parameter to the kernel.

Instead, let's add a new EFI_DISABLE_RUNTIME boolean Kconfig option, that
would be set to n by default but to y if CONFIG_PREEMPT_RT is enabled.

That way, the current behaviour is preserved but gives users a mechanism
to enable the EFI runtimes services in their kernels if that is required.
For example, if the firmware could guarantee bounded time for EFI calls.

Also, having a separate boolean config could allow users to disable the
EFI runtime services by default even when CONFIG_PREEMPT_RT is not set.

Reported-by: Alexander Larsson <alexl@redhat.com>
Fixes: d9f283ae71af ("efi: Disable runtime services on RT")
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20220331151654.184433-1-javierm@redhat.com
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>