message: revise entry code to use only assembly code in the naked function
  use suggestions of the Rust documentation
diff: @@ -11,7 +11,7 @@ use crate::boot_processor_main;
  use crate::config::KERNEL_STACK_SIZE;
  use crate::x86::controlregs::*;
  -pub unsafe fn cr0_enable_caching() {
  +unsafe fn cr0_enable_caching() {
  let mut cr0 = cr0();
  // Enable caching.
  @@ -29,13 +29,17 @@ pub unsafe extern "C" fn _start(boot_info: &'static mut BootInfo) -> ! {
  :: "r"(boot_info.current_stack_address + KERNEL_STACK_SIZE as u64 - 0x10)
  :: "volatile");
  - BOOT_INFO = boot_info as *mut BootInfo;
  + pre_init(boot_info);
  +}
  +unsafe fn pre_init(boot_info: &'static mut BootInfo) -> ! {
  //
  // CR0 CONFIGURATION
  //
  cr0_enable_caching();
  + BOOT_INFO = boot_info as *mut BootInfo;
  +
  if boot_info.cpu_online == 0 {
  boot_processor_main();
  } else {
  