rpc: WORKAROUND: disable metal_mutex

Enabling them crashes the m7 fw with this stacktrace

 #0  HardFault_Handler () at src/stm32h7xx_it.c:47
 #1  <signal handler called>
 #2  0x08004770 in __metal_mutex_acquire (mutex=0x24003318 <rvdev+88>)
    at libraries/openamp_arduino/src/metal/system/generic/mutex.h:60
 #3  metal_mutex_acquire (mutex=0x24003318 <rvdev+88>)
    at libraries/openamp_arduino/src/metal/mutex.h:59
 #4  rpmsg_virtio_rx_callback (vq=0x240070d0)
    at libraries/openamp_arduino/src/rpmsg_virtio.c:548
 #5  0x080041da in rproc_virtio_notified (vdev=0x24007058,
    notifyid=notifyid@entry=0)
    at libraries/openamp_arduino/src/remoteproc_virtio.c:341
 #6  0x08003b48 in MAILBOX_Poll (vdev=<optimized out>)
    at libraries/openamp_arduino/src/mailbox_hsem_if.c:122
 #7  0x08003d1e in OPENAMP_check_for_message ()
    at libraries/openamp_arduino/src/openamp.c:166
 #8  0x080013e2 in HSEM1_IRQHandler () at src/rpc.c:116
 #9  <signal handler called>
 #10 0x080013a8 in serial_rpc_ready () at src/rpc.c:97
 #11 0x08003596 in try_execute_m4_app () at src/m4_util.c:74
 #12 0x0800088a in main () at src/main.c:124

Investigate a proper fix

libraries/openamp_arduino/src/metal/system/generic/mutex.h

@@ -29,6 +29,14 @@ typedef struct {
 	};
 } metal_mutex_t;
 
+#define __metal_mutex_acquire(x)
+#define __metal_mutex_try_acquire(x) 0
+#define __metal_mutex_release(x)
+#define __metal_mutex_is_acquired(x)	0
+#define __metal_mutex_init(x)
+#define __metal_mutex_deinit(x)
+
+#if 0
 /*
  * METAL_MUTEX_INIT - used for initializing an mutex elmenet in a static struct
  * or global
@@ -71,6 +79,7 @@ static inline int __metal_mutex_is_acquired(metal_mutex_t *mutex)
 {
 	return atomic_load(&mutex->v);
 }
+#endif
 
 #ifdef __cplusplus
 }