Add UART echo test.

uart/README.md

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+UART Echo Test
+==============
+
+This is a trivial UART loopback program for the Sonata board. It configures the
+pinmux to conect uart1 to PMOD0 pins 2 (TX) and 3 (RX) and then enters a loop
+reading from the UART and echoing back any characters received. The baud rate is
+set to 9600. 
+
+The PMOD connector is the 15 x 2 right angled connector with holes. It is
+labelled PMOD0 on the left side (when facing the holes) and PMOD1 on the other.
+The pins used for the UART can be found like so:
+
+```
+P +--------------------------------------
+M | 3V | GD |  4 | RX | TX |  1 | ...
+O +--------------------------------------
+D | 3V | GD |  8 |  7 |  6 |  5 | ...
+0 +--------------------------------------
+```

uart/uart.cc

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+// Copyright Microsoft and CHERIoT Contributors.
+// SPDX-License-Identifier: MIT
+
+#include "compartment-macros.h"
+#include "platform/sunburst/platform-pinmux.hh"
+#include <compartment.h>
+#include <debug.hh>
+
+/// Expose debugging features unconditionally for this compartment.
+using Debug = ConditionalDebug<true, "uart">;
+
+/// Thread entry point.
+void __cheri_compartment("uart") uart_entry()
+{
+	Debug::log("Configuring pinmux");
+	auto pinSinks = MMIO_CAPABILITY(SonataPinmux::PinSinks, pinmux_pins_sinks);
+	pinSinks->get(SonataPinmux::PinSink::pmod0_2).select(4); // uart1 tx -> pmod0_2
+	auto blockSinks = MMIO_CAPABILITY(SonataPinmux::BlockSinks, pinmux_block_sinks);
+	blockSinks->get(SonataPinmux::BlockSink::uart_1_rx).select(5); // pmod0_3 -> uart1 rx
+
+	auto uart1 = MMIO_CAPABILITY(Uart, uart1);
+	uart1->init(9600);
+	while(true) {
+		char c = uart1->blocking_read();
+		Debug::log("read {}", c);
+		uart1->blocking_write(c);
+	}
+}

uart/xmake.lua

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+-- Copyright Microsoft and CHERIoT Contributors.
+-- SPDX-License-Identifier: MIT
+
+set_project("CHERIoT UART demo")
+sdkdir = "../cheriot-rtos/sdk"
+includes(sdkdir)
+set_toolchains("cheriot-clang")
+
+option("board")
+    set_default("sonata-1.1")
+
+compartment("uart")
+    -- memcpy
+    add_deps("freestanding", "debug")
+    add_files("uart.cc")
+
+-- Firmware image for the example.
+firmware("uart-demo")
+    add_deps("uart")
+    on_load(function(target)
+        target:values_set("board", "$(board)")
+        target:values_set("threads", {
+            {
+                compartment = "uart",
+                priority = 1,
+                entry_point = "uart_entry",
+                stack_size = 0x800,
+                trusted_stack_frames = 4
+            }
+        }, {expand = false})
+    end)