ivory-bsp-stm32: ringbuffer extended with empty method
ivory-bsp-stm32: UART transmit no longer requires explicit flush

note there still may be a bug related to txe not firing until after the first
rxne unless you enable txeie in the peripheral init - this bug was not present
in the old HWF4 driver.

anyway it works
ivory-bsp-tests: UART test app should not write until after periph inited