Eliminate "no previous prototype for main" warnings.

sys/boot/arm/at91/boot0/main.c

@@ -30,6 +30,8 @@
 
 typedef void fn_t(void);
 
+int main(void);
+
 int
 main(void)
 {

sys/boot/arm/at91/boot0iic/main.c

@@ -28,6 +28,8 @@
 #include "lib.h"
 #include "at91rm9200_lowlevel.h"
 
+int main(void);
+
 int
 main(void)
 {

sys/boot/arm/at91/boot0spi/main.c

@@ -31,7 +31,9 @@
 
 #define OFFSET 0
 
-void
+int main(void);
+
+int
 main(void)
 {
 	int len, i, j, off, sec;
@@ -59,4 +61,5 @@ main(void)
 	    continue;
 	printf("Done\n");
 	reset();
+	return (1);
 }

sys/boot/arm/at91/boot2/boot2.c

@@ -93,6 +93,7 @@ static char kname[1024];
 static uint32_t opts;
 static uint8_t dsk_meta;
 
+int main(void);
 static void load(void);
 static int parse(void);
 static int dskread(void *, unsigned, unsigned);
@@ -190,6 +191,7 @@ main(void)
 		else
 			load();
 	}
+	return (1);
 }
 
 static void

sys/boot/arm/at91/bootiic/main.c

@@ -28,6 +28,8 @@
 #include "emac.h"
 #include "lib.h"
 
+int main(void);
+
 /*
  * .KB_C_FN_DEFINITION_START
  * int main(void)

sys/boot/arm/at91/bootspi/main.c

@@ -43,6 +43,8 @@
 #include "spi_flash.h"
 #include "ee.h"
 
+int main(void);
+
 int
 main(void)
 {