oe.external: run in topdir, not tmpdir
If we run in tmpdir, and run a bitbake -e after wiping tmpdir,
EXTERNAL_TOOLCHAIN_SYSROOT will be 'UNKNOWN' rather than the correct path, due
to PWD not existing when we try to extract the sysroot path.
Signed-off-by: Christopher Larson <chris_larson@mentor.com>