AP_HAL: create AP_HAL namespace and use for some HAL functionality

For certain basic functionality, there aren't much benefit to be able to
vary the implementation easily at runtime. So instead of using virtual
functions, use regular functions that are "resolved" at link time. The
implementation of such functions is provided per board/platform.

Examples of functions that fit this include: getting the current
time (since boot), panic'ing, getting system information, rebooting.

These functions are less likely to benefit from the indirection provided
by virtual interfaces. For more complex hardware access APIs the
indirection makes more sense and ease the testing (when we have it!).

The idea is that instead of calling

    hal.scheduler->panic("on the streets of london");

now use

    AP_HAL::panic("on the streets of london");

A less important side-effect is that call-site code gets
smaller. Currently the compiler needs to get the hal, get the scheduler
pointer, get the right function pointer in the vtable for that
scheduler. And the call must include an extra parameter ("this"). Now it
will be just a function call, with the address resolved at link time.

This patch introduces the first functions that will be in the namespace,
further patches will implementations for each board and then switch the
call-sites. The extra init() function allow any initial setup needed for
the functions to work.

libraries/AP_HAL/AP_HAL.h

@@ -1,6 +1,4 @@
-
-#ifndef __AP_HAL_H__
-#define __AP_HAL_H__
+#pragma once
 
 #include <stdint.h>
 #include <stdbool.h>
@@ -30,5 +28,4 @@
 /* HAL Class definition */
 #include "HAL.h"
 
-#endif // __AP_HAL_H__
-
+#include "system.h"

libraries/AP_HAL/HAL.h

@@ -11,6 +11,7 @@
 #include "SPIDriver.h"
 #include "Storage.h"
 #include "UARTDriver.h"
+#include "system.h"
 
 class AP_HAL::HAL {
 public:
@@ -49,7 +50,9 @@ public:
         rcout(_rcout),
         scheduler(_scheduler),
         util(_util)
-    {}
+    {
+        AP_HAL::init();
+    }
 
     struct Callbacks {
         virtual void setup() = 0;

libraries/AP_HAL/system.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <stdint.h>
+
+#include <AP_Common/AP_Common.h>
+
+#include "AP_HAL_Macros.h"
+
+namespace AP_HAL {
+
+void init();
+
+void panic(const char *errormsg, ...) FORMAT(1, 2) NORETURN;
+
+uint32_t micros();
+uint32_t millis();
+uint64_t micros64();
+uint64_t millis64();
+
+} // namespace AP_HAL

libraries/AP_HAL_FLYMAPLE/system.cpp

@@ -0,0 +1,9 @@
+#include <AP_HAL/system.h>
+
+namespace AP_HAL {
+
+void init()
+{
+}
+
+} // namespace AP_HAL

libraries/AP_HAL_Linux/system.cpp

@@ -0,0 +1,9 @@
+#include <AP_HAL/system.h>
+
+namespace AP_HAL {
+
+void init()
+{
+}
+
+} // namespace AP_HAL

libraries/AP_HAL_PX4/system.cpp

@@ -0,0 +1,9 @@
+#include <AP_HAL/system.h>
+
+namespace AP_HAL {
+
+void init()
+{
+}
+
+} // namespace AP_HAL

libraries/AP_HAL_SITL/system.cpp

@@ -0,0 +1,9 @@
+#include <AP_HAL/system.h>
+
+namespace AP_HAL {
+
+void init()
+{
+}
+
+} // namespace AP_HAL

libraries/AP_HAL_VRBRAIN/system.cpp

@@ -0,0 +1,9 @@
+#include <AP_HAL/system.h>
+
+namespace AP_HAL {
+
+void init()
+{
+}
+
+} // namespace AP_HAL