net: lwm2m: Add LWM2M_ON_INIT() macro

Add macro that allows registration of initialization functions that
are called when LwM2M engine starts.

On LwM2M engine starts up, it first executes all initialization
functions in following priority order:
1. LWM2M_PRIO_ENGINE
2. LWM2M_PRIO_CORE, this is where all LwM2M core objects are initialized
3. LWM2M_PRIO_OBJ, this is where all other objects are initialized
4. LwM2M_PRIO_APP, application initialization.

Now on the initialization phase, we could rely that certain objects have
already been registered.
For example custom objects can register callbacks to core objects.
On application phase, we can initialize sensor objects and register
their callbacks because objects have already been initialized.

This LWM2M_ON_INIT() should replace all use of SYS_INIT()
with the default CONFIG_KERNEL_INIT_PRIORITY_DEFAULT.

Priority order is actually just alphabetical order of names, so
the order is set on a linkin phase, and we don't need any
runtime checking for it.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>

tests/net/lib/lwm2m/lwm2m_engine/CMakeLists.txt

@@ -10,6 +10,7 @@ set(APP_SRC_DIR ${CMAKE_CURRENT_SOURCE_DIR}/src)
 target_sources(app PRIVATE ${APP_SRC_DIR}/main.c)
 target_sources(app PRIVATE ${APP_SRC_DIR}/stubs.c)
 target_sources(app PRIVATE ${ZEPHYR_BASE}/subsys/net/lib/lwm2m/lwm2m_engine.c)
+zephyr_linker_sources(SECTIONS ${ZEPHYR_BASE}/subsys/net/lib/lwm2m/iterables.ld)
 
 # Add includes directories
 target_include_directories(app PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/src)

tests/net/lib/lwm2m/lwm2m_registry/src/lwm2m_registry.c

@@ -612,7 +612,6 @@ ZTEST(lwm2m_registry, test_resource_cache)
 	struct lwm2m_time_series_elem e;
 
 	/* Resource cache is turned off */
-	zassert_equal(lwm2m_engine_data_cache_init(), 0);
 	zassert_is_null(lwm2m_cache_entry_get_by_object(&path));
 	zassert_equal(lwm2m_enable_cache(&path, &e, 1), -ENOTSUP);
 	/* deprecated */