Fix error by envsetup.sh when reuse a terminal

Fix error by envsetup.sh when reuse a terminal

When reuse a terminal between defferent targets, poky's setup script
report error and setup is terminated.

E.g. Setup for qemu first, then setup for porter.
  $ source meta-agl/scripts/envsetup.sh qemux86-64 build-qemu
  $ (bitbake or somthing)
  $ cd ..
  $ source meta-agl/scripts/envsetup.sh porter build-porter
  ERROR: OE-core's config sanity cheker detected a potential misconfiguration.
  ...

Change-Id: Ie8e6f723d447b1ea53ada333f03bbfb4f03433c4
Signed-off-by: Tadao Tanikawa <tanikawa.tadao@jp.panasonic.com>