terminal.bbclass, oe-buildenv-internal: pass SCREENDIR environment variable

terminal.bbclass, oe-buildenv-internal: pass SCREENDIR environment variable

Some versions of the screen utility provided from the host OS vendor
write the socket directory to $HOME/.screen.  When using a shared home
directory across many servers, one sets the SCREENDIR environment
variable to avoid collisions in the shared home directory.  This
results in problems launching a devshell where it is not entirely
obvious what happened because the SCREENDIR environment variable
got stripped from the environment prior to setting up the screen
in detached mode.

Example:
   % bitbake -c devshell busybox
   # ...Please connect in another terminal with "screen -r devshell"

   % screen -r devshell
   There is no screen to be resumed matching devshell.

The temporary work around was to do something like:
   sh -c "unset SCREENDIR; screen -r devshell"

This patch adds SCREENDIR to the white list to ensure screen
works properly on systems where a developer needs to use
the SCREENDIR with shared home directories.

(From OE-Core rev: 5568a8f5a1c65bae021b2e36d735d3153acc6d72)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>