[kernel] Revive the debugcon logger as a kernel thread

The debugcon logger is now separate from logger::output, and is instead
a kernel-internal thread that watches for logs and prints them to the
deubcon device.

src/kernel/debugcon.cpp

@@ -0,0 +1,66 @@
+#include <util/format.h>
+
+#include "debugcon.h"
+#include "interrupts.h"
+#include "logger.h"
+
+namespace debugcon {
+
+namespace {
+    const char *level_names[] = {"", "fatal", "error", "warn", "info", "verbose", "spam"};
+    const char *area_names[] = {
+#define LOG(name, lvl) #name ,
+#include <j6/tables/log_areas.inc>
+#undef LOG
+        nullptr
+    };
+
+    inline void debug_out(const char *msg, size_t size) {
+        asm ( "rep outsb;" :: "c"(size), "d"(port), "S"(msg) );
+    }
+
+    inline void debug_newline() {
+        static const char *newline = "\r\n";
+        asm ( "rep outsb;" :: "c"(2), "d"(port), "S"(newline) );
+    }
+} // anon namespace
+
+void
+output(logs area, log::level level, const char *message, size_t mlen)
+{
+    char buffer [256];
+    size_t dlen = util::format({buffer, sizeof(buffer)}, "%7s %7s| ",
+            area_names[static_cast<uint8_t>(area)],
+            level_names[static_cast<uint8_t>(level)]);
+    debug_out(buffer, dlen);
+
+    debug_out(message, mlen);
+    debug_newline();
+}
+
+void
+logger_task()
+{
+    using entry = log::logger::entry;
+
+    uint64_t seen = 0;
+    size_t buf_size = 128;
+    uint8_t *buf = new uint8_t [buf_size];
+    entry *header = reinterpret_cast<entry*>(buf);
+
+    while (true) {
+        size_t size = g_logger.get_entry(seen, buf, buf_size);
+        if (size > buf_size) {
+            delete [] buf;
+            buf_size *= 2;
+            buf = new uint8_t [buf_size];
+            header = reinterpret_cast<entry*>(buf);
+            continue;
+        }
+
+        output(header->area, header->severity, header->message, size - sizeof(entry));
+        seen = header->id;
+    }
+}
+
+} // namespace debugcon