esp32c3: add support for this chip

This change adds support for the ESP32-C3, a new chip from Espressif. It
is a RISC-V core so porting was comparatively easy.

Most peripherals are shared with the (original) ESP32 chip, but with
subtle differences. Also, the SVD file I've used gives some
peripherals/registers a different name which makes sharing code harder.
Eventually, when an official SVD file for the ESP32 is released, I
expect that a lot of code can be shared between the two chips.

More information: https://www.espressif.com/en/products/socs/esp32-c3

TODO:
  - stack scheduler
  - interrupts
  - most peripherals (SPI, I2C, PWM, etc)

targets/esp32c3.json

@@ -0,0 +1,16 @@
+{
+	"inherits": ["riscv32"],
+	"features": ["+c", "+m"],
+	"build-tags": ["esp32c3", "esp"],
+	"scheduler": "none",
+	"serial": "uart",
+	"rtlib": "compiler-rt",
+	"libc": "picolibc",
+	"linkerscript": "targets/esp32c3.ld",
+	"extra-files": [
+		"src/device/esp/esp32c3.S"
+	],
+	"binary-format": "esp32c3",
+	"flash-command": "esptool.py --chip=esp32c3 --port {port} write_flash 0x0 {bin}"
+}
+