sandbox: Return error code from read/write/seek

The existing API for these functions is different from the rest of
U-Boot, in that any error code must be obtained from the errno variable
on failure. This variable is part of the C library, so accessing it
outside of the special 'sandbox' shim-functions is not ideal.

Adjust the API to return an error code, to avoid this. Update existing
uses to check for any negative value, rather than just -1.

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

include/os.h

@@ -29,7 +29,7 @@ int os_printf(const char *format, ...);
  * @fd:		File descriptor as returned by os_open()
  * @buf:	Buffer to place data
  * @count:	Number of bytes to read
- * Return:	number of bytes read, or -1 on error
+ * Return:	number of bytes read, or -errno on error
  */
 ssize_t os_read(int fd, void *buf, size_t count);
 
@@ -39,7 +39,7 @@ ssize_t os_read(int fd, void *buf, size_t count);
  * @fd:		File descriptor as returned by os_open()
  * @buf:	Buffer containing data to write
  * @count:	Number of bytes to write
- * Return:	number of bytes written, or -1 on error
+ * Return:	number of bytes written, or -errno on error
  */
 ssize_t os_write(int fd, const void *buf, size_t count);
 
@@ -49,7 +49,7 @@ ssize_t os_write(int fd, const void *buf, size_t count);
  * @fd:		File descriptor as returned by os_open()
  * @offset:	File offset (based on whence)
  * @whence:	Position offset is relative to (see below)
- * Return:	new file offset
+ * Return:	new file offset, or -errno on error
  */
 off_t os_lseek(int fd, off_t offset, int whence);