buildroot

Improve tar check if bsdtar is installed

If bsdtar is installed, fix script error for tar version detection.

bsdtar does not provide all expected command line (long) options
like "--hard-dereference". To ensure compatibility, mark version
of tar as 'invalid' and trigger build of 'host-tar'.

[Peter; slightly reworded commit text]
Signed-off-by: Heiko Abraham <abrahamh@web.de>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>