This follows the pattern of other peripherals (I2C, SPI) to specify the
pins using pin objects instead of a pair of GPIO port and pin number. It
makes it easier to customise the UART pins for a particular board.
All board config macros now begin with MICROPY_HW_.
Renamed PYBv10 to PYBV10, since macros should be all uppercase.
Made SDCARD_DETECT configurable in mpconfigport.h, so that the SD
detect pin can be easily configured.