[uart] Add first pass UART driver and logger

First attempt at a UART driver. I'm not sure it's the most stable. Now
that userspace is handling displaying logs, also removed serial and log
output support from the kernel.

src/user/drv.uart/serial.h

@@ -0,0 +1,33 @@
+#pragma once
+/// \file serial.h
+/// Declarations related to serial ports.
+#include <stdint.h>
+
+#include <util/bip_buffer.h>
+#include <util/spinlock.h>
+
+class serial_port
+{
+public:
+    /// Constructor.
+    /// \arg port   The IO address of the serial port
+    serial_port(uint16_t port,
+            size_t in_buffer_len, uint8_t *in_buffer,
+            size_t out_buffer_len, uint8_t *out_buffer);
+
+    size_t write(const char *str, size_t len);
+    char read();
+
+    void handle_interrupt();
+
+private:
+    bool m_writing;
+    uint16_t m_port;
+    util::bip_buffer m_out_buffer;
+    util::bip_buffer m_in_buffer;
+    util::spinlock m_lock;
+
+    void do_read();
+    void do_write();
+    void handle_error(uint16_t reg, uint8_t value);
+};