Serial Upper Half:  Add watermarks to RX flow control logic
Updte ChangeLog
Eliminate an unnecessary calculation in the serial RX flow control logic
Serial RX Flow Control:  Fix a bunch of compile problems introduced into unbuilt, conditioned out logic
Remove STM32-specific RX flow control logic from the upper level serial driver to the lower level STM32 serial driver
Integration of RTS flow control N.B. With the load as is and the mav reader task reading 32 bytes per 10 ms, it is vary hard to get nRST to go inactive (busy). If the buffer is shrunk to small i.e. less then 32 bytes the mav reader loop will never parse a block. In addtion if the High Watermark is set too low the sender may not send enough data to meet the greater then 32 requierment of the mav reader