Revert "kernel: init: activate FPU for main thread"

Activating K_FP_REGS flags introduces stack memory
overhead for the main thread in Cortex-M architecture.
Several ARM platforms experience main thread stack
overflows when building with FPU_SHARING=y.
Enabling FPU sharing in main thread should not be
the default configuration. Users are welcome to
enable FP sharing on the main thread in the
application code, in main().

This reverts commit 8453a73.

Signed-off-by: Ioannis Glaropoulos <[email protected]>

Author: ioannisg

kernel/init.c

@@ -241,7 +241,6 @@ static void init_idle_thread(int i)
 static char *prepare_multithreading(void)
 {
 	char *stack_ptr;
-	uint32_t opt;
 
 	/* _kernel.ready_q is all zeroes */
 	z_sched_init();
@@ -258,18 +257,11 @@ static char *prepare_multithreading(void)
 	 */
 	_kernel.ready_q.cache = &z_main_thread;
 #endif
-
-	opt = K_ESSENTIAL;
-#if defined(CONFIG_FPU) && defined(CONFIG_FPU_SHARING)
-	/* Enable FPU in main thread */
-	opt |= K_FP_REGS;
-#endif
-
 	stack_ptr = z_setup_new_thread(&z_main_thread, z_main_stack,
 				       CONFIG_MAIN_STACK_SIZE, bg_thread_main,
 				       NULL, NULL, NULL,
 				       CONFIG_MAIN_THREAD_PRIORITY,
-				       opt, "main");
+				       K_ESSENTIAL, "main");
 	z_mark_thread_as_started(&z_main_thread);
 	z_ready_thread(&z_main_thread);