Rework how the devshell functions

Rework how the devshell functions

In the new implementation, each known terminal is defined as a class in
oe.terminal, as a subclass of bb.process.Popen. It obeys the OE_TERMINAL
variable, which is a 'choice' typed variable. This variable may be 'auto',
'none', or any of the names of the defined terminals.

When using 'auto', or requesting an unsupported terminal, we attempt to spawn
them in priority order until we get one that's available on this system (and
in the case of the X terminals, has DISPLAY defined).  The 'none' value is
used when we're doing things like automated builds, and want to ensure that no
terminal is *ever* spawned, under any circumstances.

Current available terminals:

    gnome
    konsole
    xterm
    rxvt
    screen

Signed-off-by: Chris Larson <chris_larson@mentor.com>