Implement DEV_UART_PRINTF()

[reticulatedpines]

We use uart_printf() quite a bit in early code debugging where DryosDebugMsg() doesn't work. But, it's annoying because it's a stub some cams don't have (not needed for real builds) so you have to guard it and it's verbose.

Some macro so real builds resolve to nothing and consciously overridden builds (ML_DEV_BUILD=y)? That way it can be left in at no cost.

qprintf() would be better in most ways but we don't always emulate far enough for this to be practical.