bitbake-prserv-tool: check file name

bitbake-prserv-tool: check file name

Fixed:
$ bitbake-prserv-tool import /tmp/1
  File "/path/to/lib/bb/parse/__init__.py", line 114, in handle(fn='/tmp/1', data=<bb.data_smart.DataSmart object at 0x2369bd0>, include=True):
                     return h['handle'](fn, data, include)
    >    raise ParseError("not a BitBake file", fn)

ParseError: ParseError in /tmp/1: not a BitBake file

But 1.conf works well, its help also shows "<file.conf>", so check the
filename and print proper error message.

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