SYSTEM_CORE_CLOCK went away in favor of FUNCONF_SYSTEM_CORE_CLOCK

APB_CLOCK is only used in the i2c_slave.h and ch32v003_SPI.h files
so the references there have been maintained.

 * No more needing to decide between printf and UART in code.
 * Better debugprintf input
 * Unified code for all clock sources.
 * TINYVECTOR now saves like another 120 bytes of space.
 * None of the `Makefile`s except `bootloader` need anything extra
 * All configuration via `funconfig.h`
 * Sane defaults.
 * `SystemInit()` also initializes debugging (UART or SWIO depending on configuration)
 * Tunable timeout on debug printf.
 * Configurable UART baud rate.
 * `funconfig.h` is automatically included by `ch32v003fun.h`
 * Paves the way for other, non-003 processors.