Prevent copying of the APDU data if possible

[sirkrypt0]

To reduce the amount of memory required on the stack, we may find a way to prevent copying the data when transmitting short APDUs.

Implementation Note

The copying happens in the tx_short_apdu function.

libmicrofido2/src/nfc.c

Lines 150 to 161 in bb3678d

	static int tx_short_apdu(
	fido_dev_t *dev,
	const iso7816_header_t *h,
	const uint8_t *payload,
	uint8_t payload_len,
	uint8_t cla_flags
	) {
	// TODO: Prevent copying if possible.
	uint8_t apdu[5 + UINT8_MAX];
	uint8_t status_word[2];
	size_t apdu_len;
	int ok = FIDO_ERR_TX;

This currently allocates (mostly, depending on the platform) 263 bytes, which may not be toooo bad.