edksetup.sh: Ensure that WORKSPACE points to the top of an edk2 checkout

edksetup.sh: Ensure that WORKSPACE points to the top of an edk2 checkout

Since WORKSPACE should point at the EDK2 tree, the right place
to check its validity is edksetup.sh, not BaseTools/BuildEnv.

This patch makes sure that BuildEnv is always invoked with a valid
WORKSPACE.  This lets us show better error messages, and ensures that
we never dirty the user's environment.

BuildEnv will maintain backwards-compatibility and, should the variable
be missing, it will be able to set a default WORKSPACE.  Only the sanity
checks will be of lower quality.

Note that the error message in SetupEnv will never be shown if
SetWorkspace sets WORKSPACE=`pwd`.

Suggested-by: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15701 6f19259b-4bc3-4df7-8a09-765794883524