RTEMS 6 has moved the legacy network support out of the main repo
to a 3rd party library. This change adds support for using the
legacy networking stack with EPICS in a backwards compatible way.
A BSP can be built with libbsd or the legacy stack and by default
EPICS 7 assumes libbsd.
This patch also fixes the LDFLAGS adding the RTEMS LDFLAGS imported
from Makefile.inc.