bitbake-prserv-tool: make help text show .conf suffix

bitbake-prserv-tool: make help text show .conf suffix

'export' will accept any output filename, but 'import' needs a '.conf'
suffix to work. Otherwise you'll get:

koen@beast:/build/v2013.12$ bitbake-prserv-tool import x.txt
ERROR: Traceback (most recent call last):
  File "/build/v2013.12/sources/bitbake/lib/bb/cookerdata.py", line 162,
in wrapped
    return func(fn, *args)
  File "/build/v2013.12/sources/bitbake/lib/bb/cookerdata.py", line 172,
in parse_config_file
    return bb.parse.handle(fn, data, include)
  File "/build/v2013.12/sources/bitbake/lib/bb/parse/__init__.py", line
100, in handle
    raise ParseError("not a BitBake file", fn)
ParseError: ParseError in x.txt: not a BitBake file

ERROR: Unable to parse x.txt: ParseError in x.txt: not a BitBake file
Importing from file x.txt failed!

(From OE-Core rev: 704878ba025fda931be6accbb74cfdb14ffb55e9)

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>