RPi4: Make chainloader more robust.

Sometimes, the RPi4 would not send a correct "OK" sequence of characters when
using `print!()`. Two separate `write_char()` calls seem to improve this
situation.

Not yet sure what the exact reason is, though.

07_uart_chainloader/README.md

@@ -298,7 +298,7 @@ diff -uNr 06_drivers_gpio_uart/src/main.rs 07_uart_chainloader/src/main.rs
  mod runtime_init;
 
  // Conditionally includes the selected `BSP` code.
-@@ -65,25 +69,48 @@
+@@ -65,25 +69,49 @@
  fn kernel_main() -> ! {
      use interface::console::All;
 
@@ -338,7 +338,8 @@ diff -uNr 06_drivers_gpio_uart/src/main.rs 07_uart_chainloader/src/main.rs
 +    size |= u32::from(bsp::console().read_char() as u8) << 24;
 +
 +    // Trust it's not too big.
-+    print!("OK");
++    bsp::console().write_char('O');
++    bsp::console().write_char('K');
 +
 +    let kernel_addr: *mut u8 = bsp::BOARD_DEFAULT_LOAD_ADDRESS as *mut u8;
 +    unsafe {

07_uart_chainloader/src/main.rs

@@ -95,7 +95,8 @@ fn kernel_main() -> ! {
     size |= u32::from(bsp::console().read_char() as u8) << 24;
 
     // Trust it's not too big.
-    print!("OK");
+    bsp::console().write_char('O');
+    bsp::console().write_char('K');
 
     let kernel_addr: *mut u8 = bsp::BOARD_DEFAULT_LOAD_ADDRESS as *mut u8;
     unsafe {

08_timestamps/README.md

@@ -348,7 +348,7 @@ diff -uNr 07_uart_chainloader/src/main.rs 08_timestamps/src/main.rs
  mod runtime_init;
 
  // Conditionally includes the selected `BSP` code.
-@@ -67,50 +63,25 @@
+@@ -67,51 +63,25 @@
 
  /// The main function running after the early init.
  fn kernel_main() -> ! {
@@ -382,7 +382,8 @@ diff -uNr 07_uart_chainloader/src/main.rs 08_timestamps/src/main.rs
 -    size |= u32::from(bsp::console().read_char() as u8) << 24;
 -
 -    // Trust it's not too big.
--    print!("OK");
+-    bsp::console().write_char('O');
+-    bsp::console().write_char('K');
 -
 -    let kernel_addr: *mut u8 = bsp::BOARD_DEFAULT_LOAD_ADDRESS as *mut u8;
 -    unsafe {