* Fixes the lack of setup time delay of an fixed-byte command that immediately follows an N-byte command.
* Extra 50ns will be inserted in-between fixed-byte commands but should be acceptable.
* tx/rx: Rename wrap_addr to start_addr for clarity
* Fix RX logic, when advancing the tail pointer, not retrieving the stored RX buffer start address but the default value instead
* Fix TX logic, when advancing the head pointer for next transmission, not retrieving the stored TX buffer end address but the default value instead
* Rename SPI-related const's to match the datasheet: RERXDATA→RRXDATA, WEGPDATA→WGPDATA
* Remove useless const's for SRAM default addresses for TX buffer
* Simplify code
* For example, if the PHY linkup is down, instead of looping until resumption of the linkup, a write operation now closes the socket for re-connection in the future
* Based on quartiq's minimq as of 933687c2e4
* In minimq applications, a socket is expected to be returned when `nal::TcpStack::open()` is called
* `MqttClient::read()`/`write()` takes away the TCP socket handle (wrapped as an `Option`) from its `RefCell`, and then calls `nal::TcpStack::read()`/`write()`; if NAL returns `nb::Error`, then the MQTT client will propagate and return the error, leaving `None` behind
* Afterwards, when `MqttClient::socket_is_connected()` gets called (e.g. while polling the interface), it will detect that the socket handle is `None`, and attempt to call `nal::TcpStack::open()`
* Since `open()` pops a socket from the array (`unused_handles`), when implementing this NAL the socket should have been pushed back to the stack, i.e. by `close()`; this prevents any future calls of `open()` from returning `NetworkError::NoSocket` due to emptiness of the array of socket handles
* `NetworkStack::connect()`:
* Add timeout for connection attempt
* Now returns the socket at TCP ESTABLISHED or CLOSED states, or after connection timeout
* Split `NetworkStack::update()` into `update()` (for controlling the clock) and `poll()` (for polling the smoltcp EthernetInterface)
* Also remove option `auto_time_update`; the main application is responsible for what values `embedded_time::clock::Clock::try_now()` should return
