Build: Get rid of the USERLAND define

Let's simplify things. There is now only KERNEL.
To see if you're on Serenity, check if __serenity__ is defined.

AK/LogStream.cpp

@@ -36,7 +36,7 @@ const LogStream& operator<<(const LogStream& stream, const void* value)
     return stream << String::format("%p", value);
 }
 
-#ifdef USERLAND
+#if defined (__serenity__) && !defined(KERNEL)
 static TriState got_process_name = TriState::Unknown;
 static char process_name_buffer[256];
 #endif
@@ -44,7 +44,7 @@ static char process_name_buffer[256];
 DebugLogStream dbg()
 {
     DebugLogStream stream;
-#ifdef USERLAND
+#if defined (__serenity__) && !defined(KERNEL)
     if (got_process_name == TriState::Unknown) {
         if (get_process_name(process_name_buffer, sizeof(process_name_buffer)) == 0)
             got_process_name = TriState::True;

AK/LogStream.h

@@ -3,7 +3,7 @@
 #include <AK/Types.h>
 #include <AK/kstdio.h>
 
-#ifdef USERLAND
+#ifndef KERNEL
 #    include <AK/ScopedValueRollback.h>
 #    include <AK/StringView.h>
 #    include <errno.h>
@@ -18,7 +18,7 @@ class StringView;
 class LogStream {
 public:
     LogStream()
-#ifdef USERLAND
+#ifndef KERNEL
         : m_errno_restorer(errno)
 #endif
     {
@@ -28,7 +28,7 @@ public:
     virtual void write(const char*, int) const = 0;
 
 private:
-#ifdef USERLAND
+#ifndef KERNEL
     ScopedValueRollback<int> m_errno_restorer;
 #endif
 };

Kernel/kstdio.h

@@ -11,7 +11,7 @@ void set_serial_debug(bool on_or_off);
 int get_serial_debug();
 }
 
-#ifndef USERLAND
+#ifdef KERNEL
 #    define printf dbgprintf
 #endif
 

Makefile.common

@@ -54,8 +54,6 @@ else
 
     ifdef KERNEL
         DEFINES += -DKERNEL
-    else
-        DEFINES += -DUSERLAND
     endif
 
     OBJ_SUFFIX ?=