Playback: Move internal track list onto buffer

Does away the statically-allocated track list which frees quite
a fair amount of in-RAM size.

There's no compile-time hard track limit.

Recommended TODO (but not right away): Have data small enough use
the handle structure as its buffer data area. Almost the entire
handle structure is unused for simple allocations without any
associated filesystem path.

Change-Id: I74a4561e5a837e049811ac421722ec00dadc0d50

firmware/export/system.h

@@ -97,6 +97,11 @@ int get_cpu_boost_counter(void);
 #define MAX(a, b) (((a)>(b))?(a):(b))
 #endif
 
+#ifndef SGN
+#define SGN(a) \
+    ({ typeof (a) ___a = (a); (___a > 0) - (___a < 0); })
+#endif
+
 /* return number of elements in array a */
 #define ARRAYLEN(a) (sizeof(a)/sizeof((a)[0]))