It is possible for a socket to have no connection, for example if a TCP
socket is waiting in accept.  This checks for this condition in case the
socket is closed from a different thread.

When a socket is closed, it should make sure that any pending write
data is sent before the FIN is sent.  It already would wait for all
sent data to be acked, however it would discard any pending write
data that had not been sent at least once.

This change adds a check for pending write data in addition to unacked
data.  However, to be able to actually send any new data, the send
callback must be left.  The callback should be freed later when the socket
is actually destroyed.

When a poll requesting POLLOUT happens, the poll should return
immediately if a write will not block.  This change adds that, as
opposed to the old behaviour of blocking until a timer from the
Ethernet driver eventually triggers the poll to complete.

This is only implemented for buffered TCP.  Unbuffered TCP should
behave as before.

If stdout is a socket (e.g. telnet), then the debug message will cause
more TCP data to be sent, which generates another message, and so on

Fix romdisk_register prototype.  Most ROMFS images actually lie in RAM.  If moved to FLASH with const storage class, then changes are required to avoid warnings

Add LSM9DS1 accelerometer, gyroscope and magnetometer

add NXP pn532 NFC-chip support

Signed-off-by: Janne Rosberg <janne.rosberg@offcode.fi>