[gem][minip][pktbuf] Improvements for TX scatter gather functionality

- GEM now supports asynchronous scatter-gather queuing and properly handling
  pktbuf ownership
- General stack cleanups in Minip
- pktbufs now grab buffers from the user, or from a preallocated pool

target/uzed/target.c

@@ -180,7 +180,7 @@ void target_early_init(void)
 
 void target_init(void)
 {
-    gem_init(GEM0_BASE, 256*1024);
+    gem_init(GEM0_BASE);
 }
 
 void target_set_debug_led(unsigned int led, bool on)