Add example for using the pre_init macro

Signed-off-by: Gabriel Smith <[email protected]>

Author: yodaldevoid

cortex-m-rt/examples/pre_init.rs

@@ -0,0 +1,36 @@
+//! `cortex-m-rt` based program with a function run before RAM is initialized.
+
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+#[macro_use(entry, exception, pre_init)]
+extern crate cortex_m_rt as rt;
+extern crate panic_semihosting;
+
+use rt::ExceptionFrame;
+
+pre_init!(disable_watchdog);
+
+unsafe fn disable_watchdog() {
+    // Do what you need to disable the watchdog.
+}
+
+// the program entry point
+entry!(main);
+
+fn main() -> ! {
+    loop {}
+}
+
+// the hard fault handler
+exception!(HardFault, hard_fault);
+
+fn hard_fault(_ef: &ExceptionFrame) -> ! {
+    loop {}
+}
+
+// the default exception handler
+exception!(*, default_handler);
+
+fn default_handler(_irqn: i16) {}