message: add some comments to boot code
diff: @@ -18,7 +18,7 @@ static mut BOOT_STACK: [u8; KERNEL_STACK_SIZE] = [0; KERNEL_STACK_SIZE];
  pub unsafe extern "C" fn _start() -> ! {
  asm!("ldr x1, {0}",
  "add x1, x1, {1}",
  - "sub x1, x1, #0x10", /*Previous version subtracted 0x10 from End, so I'm doing this too. Not sure why though */
  + "sub x1, x1, #0x10", /* Previous version subtracted 0x10 from End, so I'm doing this too. Not sure why though. COMMENT from SL: This is a habit of mine. I always start 16 bytes before the end of the stack. */
  "mov sp, x1",
  /* Set exception table */
  "adr x8, vector_table",
  