uefi: _print more failsafe

When one accidentally call println! after exit_boot_services, one now at least
gets some more debugging info why the machine suddenly reboots.

Specifically, this prints now info to `integration-test-debugcon.log` in such
a case.

Author: phip1611

uefi/src/helpers/println.rs

@@ -4,14 +4,25 @@ use core::fmt::Write;
 /// INTERNAL API! Helper for print macros.
 #[doc(hidden)]
 pub fn _print(args: core::fmt::Arguments) {
-    system_table_boot()
-        .expect("boot services are not active")
-        .stdout()
-        .write_fmt(args)
-        .expect("Failed to write to stdout");
+    if let Some(mut bs) = system_table_boot() {
+        bs.stdout()
+            .write_fmt(args)
+            .expect("Failed to write to stdout");
+    } else {
+        // Ease debugging: Depending on logger, this might write to serial or
+        // debugcon.
+        log::debug!("You are using `print!` after the boot services have been exited.");
+    }
 }
 
-/// Prints to the standard output.
+/// Prints to the standard output of the UEFI boot service console. 
+/// 
+/// # Usage
+/// Use this similar to `print!` from the Rust standard library, but only 
+/// as long as boot services have not been exited.
+/// 
+/// You should never use this macro in a custom Logger ([`log::Log`] impl) to
+/// prevent a circular runtime dependency.
 ///
 /// # Panics
 /// Will panic if `SYSTEM_TABLE` is `None` (Before [`uefi::helpers::init()`] and
@@ -28,7 +39,15 @@ macro_rules! print {
     ($($arg:tt)*) => ($crate::helpers::_print(core::format_args!($($arg)*)));
 }
 
-/// Prints to the standard output, with a newline.
+/// Prints to the standard output of the UEFI boot service console, but with a 
+/// newline.
+///
+/// # Usage
+/// Use this similar to `println!` from the Rust standard library, but only 
+/// as long as boot services have not been exited.
+///
+/// You should never use this macro in a custom Logger ([`log::Log`] impl) to
+/// prevent a circular runtime dependency.
 ///
 /// # Panics
 /// Will panic if `SYSTEM_TABLE` is `None` (Before [`uefi::helpers::init()`] and