Loop for non-0 hartid's

src/main.rs

@@ -14,6 +14,8 @@ mod uart;
 unsafe extern "C" fn _enter() -> ! {
     use core::arch::asm;
     asm!(
+        "csrr t0, mhartid",
+        "bnez t0, 3f",
         // before we use the `la` pseudo-instruction for the first time,
         // we need to set `gp` (google linker relaxation)
         ".option push", // pushes the current option stack
@@ -38,6 +40,11 @@ unsafe extern "C" fn _enter() -> ! {
         "bne t0, t1, 1b",
     "2:",
         // BSS is clear!
+        "j 4f",
+    "3:",
+        "wfi",
+        "j 3b",
+    "4:",
 
         // "tail-call" to {start} (call without saving a return address)
         "tail {start}",