bitbake.conf: replace USE_LDCONFIG with new "ldconfig" distro feature

bitbake.conf: replace USE_LDCONFIG with new "ldconfig" distro feature

USE_LDCONFIG could previously be set to 0 by distros which do not
require ldconfig or ld.so.conf on the target. Since more and more
recipes may need to respect that option, replace the ad-hoc variable
with a distro feature.

Distros which previously set:

  USE_LDCONFIG = "0"

Should now instead use:

  DISTRO_FEATURES_BACKFILL_CONSIDERED_append = " ldconfig"

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>