make config.mk.example more useful
ivory-bsp-stm32f4: i2c debugging extended
ivory-bsp-stm32f4: max syscall priority specified in single spot
ivory-bsp-stm32f4: bugfix in i2c driver: never set the isr priority

- this one stumped me for days, ashamed to say
ivory-px4-hw: i2c driver fixed, ms5611 test now works w/o dead uart driver