efi_loader: prepare for read only OP-TEE variables

We currently have two implementations of UEFI variables:

* variables provided via an OP-TEE module
* variables stored in the U-Boot environment

Read only variables are up to now only implemented in the U-Boot
environment implementation.

Provide a common interface for both implementations that allows handling
read-only variables.

As variable access is limited to very few source files put variable
related definitions into new include efi_variable.h instead of efi_loader.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/api/efi.rst b/doc/api/efi.rst
index d5114f0..cb2a1c8 100644
--- a/doc/api/efi.rst
+++ b/doc/api/efi.rst
@@ -93,6 +93,8 @@
 Variable services
 ~~~~~~~~~~~~~~~~~
 
+.. kernel-doc:: include/efi_variable.h
+   :internal:
 .. kernel-doc:: lib/efi_loader/efi_variable.c
    :internal: