constrain all _fk joints to respective _bn assuming root_bn exists.

forEachConstrain.py

import maya.cmds as cmds

# select all joints _bn
cmds.select('root_bn', hierarchy=True)

# For each _fk constrain to _bn
for bn in cmds.ls(selection=True):
    try:
        cmds.orientConstraint( bn.replace('_bn', '_fk'), bn )
        cmds.pointConstraint( bn.replace('_bn', '_fk'), bn )
    except Exception as e:
        print e.args