AP_HAL_Linux: delay signal handlers setup

Allow default signals before full initialization in Linux, this makes sure we don't get an unkillable process if it hangs on initialization

Exit flag marked volatile to counteract possible compiler optimization due to the handler code running in a different context

libraries/AP_HAL_Linux/HAL_Linux_Class.cpp

@@ -467,7 +467,11 @@ void HAL_Linux::run(int argc, char* const argv[], Callbacks* callbacks) const
         }
     }
 
-    setup_signal_handlers();
+    // NOTE: signal handlers are only set before the main loop, so
+    // that if anything before the loops hangs, the default signals
+    // can still stop the process proprely, although without proper
+    // teardown.
+    // This isn't perfect, but still prevents an unkillable process.
 
     scheduler->init();
     gpio->init();
@@ -497,6 +501,8 @@ void HAL_Linux::run(int argc, char* const argv[], Callbacks* callbacks) const
     AP_Module::call_hook_setup_complete();
 #endif
 
+    setup_signal_handlers();
+
     while (!_should_exit) {
         callbacks->loop();
     }

libraries/AP_HAL_Linux/HAL_Linux_Class.h

@@ -2,6 +2,8 @@
 
 #include <AP_HAL/AP_HAL.h>
 
+#include <signal.h>
+
 class HAL_Linux : public AP_HAL::HAL {
 public:
     HAL_Linux();
@@ -12,7 +14,7 @@ public:
     static void exit_signal_handler(int);
 
 protected:
-    bool _should_exit = false;
+    volatile sig_atomic_t _should_exit = false;
 };
 
 #if HAL_NUM_CAN_IFACES