efi_loader: optional persistence of variables

Since be66b89d ("efi_loader: configuration of variables store")
the choice of EFI_VARIABLE_FILE_STORE or EFI_MM_COMM_TEE
is mutually-exclusive, however efi_var_to_file also allows
for "neither". Provide an additional Kconfig option.

Signed-off-by: Tom Saeger <tom.saeger@oracle.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>