Add 3rd hardware revision

* Replace single resistors with resistor networks.
* Add pull-ups to the uSD SPI signals and unused pins.
* Add a 2mm pitch debugging header.
* Breakout I2C3, SPI3 and USART3 using the main header.
* Fix silk screen component names.
* Connect the sensor's PWDWN and RESET to GPIO.
* Add a MOSFET load switch to enable/disable power to the uSD.
* Add a bulk cap to STM micro.
* Add a switch to enter the DFU mode.