zephyr: Link MicroPython with the Zephyr kernel library.

Unlike most other Zephyr libraries, libkernel.a is not built as a
whole-archive.

This change also fixes a linker error observed on nucleo_wb55rg while
preparing an upgrade to Zephyr v3.5.0, caused by an undefined reference
to `z_impl_k_busy_wait'.

Signed-off-by: Maureen Helm <maureen.helm@analog.com>

ports/zephyr/CMakeLists.txt

@@ -109,6 +109,7 @@ zephyr_library_compile_definitions(
 )
 
 zephyr_library_sources(${MICROPY_SOURCE_QSTR})
+zephyr_library_link_libraries(kernel)
 
 add_dependencies(${MICROPY_TARGET} zephyr_generated_headers)
 

ports/zephyr/src/zephyr_start.c

@@ -36,8 +36,4 @@ void main(void) {
     zephyr_getchar_init();
     #endif
     real_main();
-
-    // This is needed so the linker includes k_timer_init, z_impl_k_timer_start
-    // and z_impl_k_timer_stop, as used by libmicropython.a.
-    k_timer_start(NULL, K_MSEC(0), K_MSEC(0));
 }