build: add --apstatedir for Linux boards

This is the --localstatedir that is found on other
buildsystems/programs + the package directory. We could provide a
--localstatedir instead to be similar to the others, but I thought it
would be too confusing for the help message.

Tools/ardupilotwaf/boards.py

@@ -437,7 +437,10 @@ class linux(Board):
 
         if self.with_uavcan:
             cfg.define('UAVCAN_EXCEPTIONS', 0)
-    
+
+        if cfg.options.apstatedir:
+            cfg.define('AP_STATEDIR', cfg.options.apstatedir)
+
     def build(self, bld):
         super(linux, self).build(bld)
         if bld.options.upload:

wscript

@@ -101,6 +101,13 @@ submodules at specific revisions.
             opt.parser.remove_option(k)
             g.add_option(option)
 
+    g.add_option('--apstatedir',
+        action='store',
+        default='',
+        help='''Where to save data like parameters, log and terrain.
+This is the --localstatedir + ArduPilot's subdirectory [default:
+board-dependent, usually /var/lib/ardupilot]''')
+
     g.add_option('--rsync-dest',
         dest='rsync_dest',
         action='store',