ivory-bsp-stm32f4: add interrupt set/clear pending to api

* didn't help my problem, but they were pretty mechanical to add
ivory-bsp-stm32f4: add timing debuggable version of uart tower
ivory-bsp-tests: using uart stress test to debug timing with logic analyzer
ivory-bsp-stm32f4: big efficiency improvement in uart driver

i was able to eliminate a lot of spurious interrupt servicing by disabling
TXEIE and EXNEIE in the ISR context and only enabling them again after
clearing any interrupt-causing conditions in the service thread