board_serial: use a uint8_t buffer

we should not be using 'char' for binary APIs, as the C standard does
not specify if it is signed or unsigned, so results may not be
consistent

src/drivers/px4fmu/fmu.cpp

@@ -1784,7 +1784,7 @@ fmu_main(int argc, char *argv[])
 	}
 
 	if (!strcmp(verb, "id")) {
-		char id[12];
+		uint8_t id[12];
 		(void)get_board_serial(id);
 
 		errx(0, "Board serial:\n %02X%02X%02X%02X %02X%02X%02X%02X %02X%02X%02X%02X",

src/modules/systemlib/board_serial.c

@@ -44,11 +44,11 @@
 #include "board_config.h"
 #include "board_serial.h"
 
-int get_board_serial(char *serialid)
+int get_board_serial(uint8_t *serialid)
 {
-	const volatile unsigned *udid_ptr = (const unsigned *)UDID_START;
+	const volatile uint32_t *udid_ptr = (const uint32_t *)UDID_START;
 	union udid id;
-	val_read((unsigned *)&id, udid_ptr, sizeof(id));
+	val_read((uint32_t *)&id, udid_ptr, sizeof(id));
 
 
 	/* Copy the serial from the chips non-write memory and swap endianess */
@@ -57,4 +57,4 @@ int get_board_serial(char *serialid)
 	serialid[8] = id.data[11];   serialid[9] = id.data[10];  serialid[10] = id.data[9];  serialid[11] = id.data[8];
 
 	return 0;
-}
+}

src/modules/systemlib/board_serial.h

@@ -44,6 +44,6 @@
 
 __BEGIN_DECLS
 
-__EXPORT int get_board_serial(char *serialid);
+__EXPORT int get_board_serial(uint8_t *serialid);
 
 __END_DECLS

src/modules/systemlib/otp.h

@@ -125,7 +125,7 @@ struct otp_lock {
 #pragma pack(push, 1)
 union udid {
 	uint32_t	serial[3];
-	char  data[12];
+	uint8_t  data[12];
 };
 #pragma pack(pop)