CM4008: fix signedness of env bootargs string pointer

The pointer to the flash based bootargs should be a "char *", not unsigned.
Fixes:

cm4008.c: In function ‘env_flash_cmdline’:
cm4008.c:67: warning: pointer targets in passing argument 2 of ‘setenv’ differ in signedness
Signed-off-by: NGreg Ungerer <greg.ungerer@opengear.com>

board/cm4008/cm4008.c

@@ -46,8 +46,8 @@ DECLARE_GLOBAL_DATA_PTR;
  */
 int env_flash_cmdline (void)
 {
-	unsigned char *sp = (unsigned char *) 0x0201c020;
-	unsigned char *ep;
+	char *sp = (char *) 0x0201c020;
+	char *ep;
 	int len;
 
 	/* Check if "erase" push button is depressed */