oe.terminal: improve how we spawn screen

- Name the screen session 'devshell', to avoid confusion if running bitbake
  itself under a screen session.
- Display a warning message when spawning screen, so it's clear to the user
  that screen has been run (otherwise do_devshell just appears to hang).

(From OE-Core rev: 13e01d29d14e7e7403d0c45c5699ea9160243868)

Signed-off-by: Chris Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oe/terminal.py

@@ -70,7 +70,12 @@ class Rxvt(XTerminal):
     priority = 1
 
 class Screen(Terminal):
-    command = 'screen -D -m -t "{title}" {command}'
+    command = 'screen -D -m -t "{title}" -S devshell {command}'
+
+    def __init__(self, command, title=None, env=None):
+        Terminal.__init__(self, command, title, env)
+        logger.warn('Screen started. Please connect in another terminal with '
+                    '"screen -r devshell"')
 
 
 def prioritized():