sandbox: Support file truncation with os_open()

At present files are not truncated on writing. This is a useful feature.
Add support for this.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/os.h

@@ -75,6 +75,7 @@ int os_open(const char *pathname, int flags);
 #define OS_O_RDWR	2
 #define OS_O_MASK	3	/* Mask for read/write flags */
 #define OS_O_CREAT	0100
+#define OS_O_TRUNC	01000
 
 /**
  * Access to the OS close() system call