Make yield a regular syscall taking an argument.

Allows "wait for interrupt"/"wait for event" type operation where VM code blocks until the host has something for it.

apps/lissajous/lissajous.c

@@ -13,13 +13,6 @@ void movecursor(int x, int y) {
     print("f");
 }
 
-void sleep(uint32_t ms) {
-    uint32_t start = millis();
-    while (millis() < start + ms) {
-        yield();
-    }
-}
-
 // Absolute value function for doubles
 double abs_c(double x) {
     if (x < 0) {
@@ -116,7 +109,7 @@ void main(void) {
 
         // wait for next frame
         while (millis() < framestart + (1000 / 30)) {
-            yield();
+            yield(0);
         }
         beta += 0.05;
         freq1 += 0.01;

apps/self/self.c

@@ -45,6 +45,8 @@ void main(void) {
             break;
             case UVM32_EVT_SYSCALL:    // vm has paused to handle UVM32_SYSCALL
                 switch(evt.data.syscall.code) {
+                    case UVM32_SYSCALL_YIELD:
+                    break;
                     case UVM32_SYSCALL_PUTC:
                         putc(uvm32_getval(&vmst, &evt, ARG0));
                     break;

apps/sketch/sketch.c

@@ -25,7 +25,7 @@ void setup(void) {
 void main(void) {
     setup();
     while(loop()) {
-        yield();
+        yield(0);
     }
 }