[include] move almost all of the remainder of top level includes into a subdir

Examples are include/platform.h -> platform/include/platform.h
include/target.h -> target/include/target.h

The old model generally considered these to be Always There includes,
but they're starting to stick out more and more so may as well actually
follow the model that most of the rest of the system follows.

target/include/target.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2008-2012 Travis Geiselbrecht
+ *
+ * Use of this source code is governed by a MIT-style
+ * license that can be found in the LICENSE file or at
+ * https://opensource.org/licenses/MIT
+ */
+#pragma once
+
+#include <lk/compiler.h>
+#include <stdbool.h>
+
+__BEGIN_CDECLS
+
+/* super early platform initialization, before almost everything */
+void target_early_init(void);
+
+/* later init, after the kernel has come up */
+void target_init(void);
+
+/* called during chain loading to make sure target specific bits are put into a stopped state */
+void target_quiesce(void);
+
+/* a target can optionally define a set of debug leds that can be used
+ * in various locations in the system.
+ */
+#if TARGET_HAS_DEBUG_LED
+void target_set_debug_led(unsigned int led, bool on);
+#else
+#define target_set_debug_led(led, on) ((void)(0))
+#endif
+
+__END_CDECLS
+