STM32 Serial: PX4 HW workarround for flaky STM32 RTS.  From David Sidrane
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
Merge branch 'nuttx_bringup_upstream_merged' of https://github.com/PX4-Works/NuttX into nuttx_bringup_upstream_merged