efi: efivars: drop kobject from efivars_register()

Since commit 0f5b2c69

 ("efi: vars: Remove deprecated 'efivars' sysfs
interface") and the removal of the sysfs interface there are no users of
the efivars kobject.

Drop the kobject argument from efivars_register() and add a new
efivar_is_available() helper in favour of the old efivars_kobject().

Note that the new helper uses the prefix 'efivar' (i.e. without an 's')
for consistency with efivar_supports_writes() and the rest of the
interface (except the registration functions).

For the benefit of drivers with optional EFI support, also provide a
dummy implementation of efivar_is_available().

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>