dsp: add sample programs

dsp/echo.c

@@ -0,0 +1,22 @@
+#include "sys.h"
+
+int strlen(const char* str) {
+  int i = 0;
+  while(str[i] != '\0') {
+    i++;
+  }
+  return i;
+}
+
+void log(const char* stuff) {
+  uart_writen(uart0, stuff, strlen(stuff));
+}
+
+int main() {
+  led0->output = 37;
+
+  while(1) {
+    uint8_t c = uart_read(uart0);
+    uart_write(uart0, c);
+  }
+}