devshell support

Add an OE-specific bash --rcfile and a tiny script to invoke it when using
bitbake's "-c devshell" option. In order to tie this into a build, include the
following lines in your conf/local.conf file:

	OE_TERMINAL = "custom"
	OE_TERMINAL_CUSTOMCMD = "/usr/bin/xterm -e /home/trevor/local/bin/oeshell.sh"

Signed-off-by: Trevor Woerner <twoerner@gmail.com>