sanity.bbclass: fix check_connectivity() for BB_NO_NETWORK = "0"

sanity.bbclass: fix check_connectivity() for BB_NO_NETWORK = "0"

The old code:
network_enabled = not d.getVar('BB_NO_NETWORK', True)

It is True only when BB_NO_NETWORK is not set (None),
but BB_NO_NETWORK = "0" should also be True while "1" means no network,
"0" means need network in a normal case.

(From OE-Core rev: 3d8db6cb992f96023a0486f64fe6b0f1ead04184)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>