If write buffering is enabled, then it is necessary to release the write buffer resources (callback structure) when the socket is closed

Add support for the SO_LINGER socket option.  Based on logic from Jason Jiang.  Utestested on initial commit

Fix STM32F103VE SPI3 definitions; Fix bug introduced in recent NX reorganization.  From Steve Redler IV

mkconfig: Cast RAM size to unsigned to avoid complains about integer overflow in calculatino of CONFIG_RAM_END