`TxFrame::GenerateEnhAck()` parses the received frame fully, which fails
if a radio platform builds the ACK mid-RX before the Header IE and MAC
Command ID bytes are in RAM (these bytes are not required for Enh-ACK
construction).
Add `OPENTHREAD_CONFIG_MAC_ALLOW_INCOMPLETE_RX_FRAME_WHEN_GEN_ENH_ACK`
so only fields through the Auxiliary SecHdr are required to be parseable.
The platform must still suppress Enh-ACKs for frames not successfully
received as per 802.15.4.