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Minimal printing facilities for ufmt

§Supported platforms:

• wasm via wasm-bindings;
• mos microprocessors via linking putchar;
• riscv32 via esp-* features (see details in Features section) or being no-op otherwise;
• All other platforms are built upon standard C library write function.

§Features

• esp-uart - Enables UART writer on riscv32 targets. Mutually exclusive with esp-jtag. Requires user to provide symbols:
  • ESP_UART_ADDR (e.g. on ESP32-C3 it is #[no_mangle] static ESP_UART_ADDR: usize = 0x40000068).
• esp-jtag - Enables JTAG writer on riscv32 targets. Mutually exclusive with esp-jtag. Requires user to provide symbols:
  • SERIAL_JTAG_FIFO_REG (e.g. on ESP32-C3 it is #[no_mangle] static SERIAL_JTAG_FIFO_REG: usize = 0x60043000)
  • SERIAL_JTAG_CONF_REG (e.g. on ESP32-C3 it is #[no_mangle] static SERIAL_JTAG_CONF_REG: usize = 0x60043004).