Extend hostapd rsnxe_override_eapol to allow IE removal

Extend hostapd rsnxe_override_eapol to allow IE removal

Previous implementation was determining whether the override value was
set based on its length being larger than zero. Replace this with an
explicit indication of whether the parameter is set to allow zero length
replacement, i.e., remove of RSNXE from EAPOL-Key msg 3/4.

In addition, move IE replacement into a more generic helper function to
allow this to be used with other IEs as well.

Signed-off-by: Jouni Malinen <j@w1.fi>