Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

drivers/alsamidi/alsa_midi.cpp

@@ -30,11 +30,12 @@
 
 #ifdef ALSAMIDI_ENABLED
 
-#include <errno.h>
-
 #include "alsa_midi.h"
-#include "os/os.h"
-#include "print_string.h"
+
+#include "core/os/os.h"
+#include "core/print_string.h"
+
+#include <errno.h>
 
 static int get_message_size(uint8_t message) {
 	switch (message & 0xF0) {

drivers/alsamidi/alsa_midi.h

@@ -33,13 +33,13 @@
 #ifndef ALSA_MIDI_H
 #define ALSA_MIDI_H
 
-#include <alsa/asoundlib.h>
-#include <stdio.h>
-
+#include "core/os/midi_driver.h"
 #include "core/os/mutex.h"
 #include "core/os/thread.h"
 #include "core/vector.h"
-#include "os/midi_driver.h"
+
+#include <alsa/asoundlib.h>
+#include <stdio.h>
 
 class MIDIDriverALSAMidi : public MIDIDriver {