populate_sdk_ext.bbclass: check unfsd before create it

populate_sdk_ext.bbclass: check unfsd before create it

Fixed when nativesdk-unfs3 is installed:
$ bitbake <image> -c populate_sdk_ext
| Traceback (most recent call last):
|   File "/path/to/oe-core/scripts/lnr", line 21, in <module>
|     os.symlink(target, linkname)
| FileExistsError: [Errno 17] File exists: '../../../../tmp/sysroots/x86_64-linux/usr/bin/unfsd' -> '/path/to/9.0/sysroots/x86_64-wrlinuxsdk-linux/usr/bin/unfsd'

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>