turn on -Werror and fix resulting errors

src/modules/systemlib/module.mk

@@ -57,3 +57,5 @@ SRCS		 = err.c \
 		   mcu_version.c
 
 MAXOPTIMIZATION	 = -Os
+
+EXTRACFLAGS	= -Wno-sign-compare

src/modules/systemlib/systemlib.c

@@ -87,7 +87,7 @@ static void kill_task(FAR struct tcb_s *tcb, FAR void *arg)
 	kill(tcb->pid, SIGUSR1);
 }
 
-int task_spawn_cmd(const char *name, int scheduler, int priority, int stack_size, main_t entry, const char *argv[])
+int task_spawn_cmd(const char *name, int scheduler, int priority, int stack_size, main_t entry, char * const argv[])
 {
 	int pid;
 

src/modules/systemlib/systemlib.h

@@ -64,7 +64,7 @@ __EXPORT int task_spawn_cmd(const char *name,
 			int scheduler,
 			int stack_size,
 			main_t entry,
-			const char *argv[]);
+			char * const argv[]);
 
 enum MULT_PORTS {
 	MULT_0_US2_RXTX = 0,