add read call

kernel/syscall/skeleton.cc

@@ -34,10 +34,17 @@ size_t write(Vault &vault, uint32_t id, const void *buffer, size_t size) {
     return 0;
 }
 
-size_t read(Vault &vault, uint32_t id) {
-	(void)vault;
-	(void)id;
-	return 0;
+size_t read(Vault &vault, uint32_t id, void *buf, size_t len) {
+    size_t read_cnt = 0;
+    while(read_cnt <= len){
+        Key key;
+        vault.keys.consume(key);
+        if(key.valid())
+            ((char*)buf)[read_cnt++] = key.ascii();
+        else
+            return read_cnt;
+    }
+    return len;
 }
 
 void sleep(Vault &vault, size_t ms) {