Restrict access to the UART when logging

Added a mutex around the UART in the console backend to prevent
multiple tasks from stomping on each other. This eliminates UART
corruption garbling the tokenized output.

target/ast1060-evb/BUILD.bazel

diff --git a/target/ast1060-evb/BUILD.bazel b/target/ast1060-evb/BUILD.bazel
index 797ac15..f26545a 100644
--- a/target/ast1060-evb/BUILD.bazel
+++ b/target/ast1060-evb/BUILD.bazel
@@ -60,6 +60,8 @@
     deps = [
         "@oot_crates_no_std//:ast1060-pac",
         "@pigweed//pw_status/rust:pw_status",
+        "@pigweed//pw_kernel/arch/arm_cortex_m:arch_arm_cortex_m",
+        "@pigweed//pw_kernel/kernel",
         "@oot_crates_no_std//:aspeed-ddk",
         "@oot_crates_no_std//:embedded-hal",
         "@oot_crates_no_std//:embedded-io",

target/ast1060-evb/console_backend.rs

diff --git a/target/ast1060-evb/console_backend.rs b/target/ast1060-evb/console_backend.rs
index 237ef76..d5c267d 100644
--- a/target/ast1060-evb/console_backend.rs
+++ b/target/ast1060-evb/console_backend.rs
@@ -12,12 +12,14 @@
 use pw_status::{Error, Result};
 use ast1060_pac::Peripherals;
 use embedded_io::Write;
+use kernel::sync::spinlock::SpinLock;
 
 use aspeed_ddk::uart::{Config, Parity, StopBits, UartController};
 
-// Global UART controller instance
+// Global UART controller instance wrapped in a spinlock
 static mut UART_CONTROLLER: MaybeUninit<UartController<'static>> = MaybeUninit::uninit();
 static UART_INITIALIZED: AtomicBool = AtomicBool::new(false);
+static UART_LOCK: SpinLock<arch_arm_cortex_m::Arch, ()> = SpinLock::new(());
 
 struct DummyDelay;
 
@@ -73,8 +75,10 @@
         return Err(Error::Unavailable);
     }
 
-    // Safety: logical singlton access pattern guarded by UART_INITIALIZED.
-    // In a real multi-threaded kernel we'd need a spinlock here.
+    // Acquire spinlock to ensure exclusive UART access
+    let _guard = UART_LOCK.lock(arch_arm_cortex_m::Arch);
+
+    // Safety: exclusive access is guaranteed by the spinlock guard above.
     let controller = unsafe {
         &mut *(core::ptr::addr_of_mut!(UART_CONTROLLER) as *mut UartController<'static>)
     };