extensible sdk: Error when trying to install as root.

extensible sdk: Error when trying to install as root.

Since the extensible sdk uses bitbake, which can't run as root, the sdk
shouldn't be installed as root.

Previously it would error out late into setup when bitbake errored
saying not to run bitbake as root.

Now the script errors with a message saying the extensible sdk can't be
installed as root.

[Yocto #7545]

Signed-off-by: Randy Witt <randy.e.witt@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>