wireless_tools: don't call ldconfig

The wireless_tools installation process calls ldconfig to update the
ld.so cache, but in a cross-compilation situation, it doesn't make
sense, and simply takes time for nothing. To avoid this, we pass
LDCONFIG=/bin/true during the wireless_tools installation steps.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>