devices: Add utility functions for the serial output buffer

In preparation for reorganizing how the serial output is constructed
add methods to the serial devices for setting the out buffer after the
device is created.

Also add a method to enable flushing the output buffer to be used to
write the buffer to the PTY fd once the PTY is writable.

Signed-off-by: William Douglas <william.douglas@intel.com>

devices/src/legacy/uart_pl011.rs

@@ -138,6 +138,10 @@ impl Pl011 {
         }
     }
 
+    pub fn set_out(&mut self, out: Box<dyn io::Write + Send>) {
+        self.out = Some(out);
+    }
+
     fn state(&self) -> Pl011State {
         Pl011State {
             flags: self.flags,
@@ -192,6 +196,13 @@ impl Pl011 {
         Ok(())
     }
 
+    pub fn flush_output(&mut self) -> result::Result<(), io::Error> {
+        if let Some(out) = self.out.as_mut() {
+            out.flush()?;
+        }
+        Ok(())
+    }
+
     fn pl011_get_baudrate(&self) -> u32 {
         if self.fbrd == 0 {
             return 0;