build: invert default for lttng

Since it's a debug option, particularly for the scheduler, leave it
disabled by default and let the user enable on demand.

Tools/ardupilotwaf/cxx_checks.py

@@ -191,13 +191,13 @@ def check_package(cfg, env, libname):
 
 @conf
 def check_lttng(cfg, env):
+    if not cfg.options.enable_lttng:
+        cfg.msg("Checking for 'lttng-ust':", 'disabled', color='YELLOW')
+        return False
     if cfg.env.STATIC_LINKING:
         # lttng-ust depends on libdl which means it can't be used in a static build
         cfg.msg("Checking for 'lttng-ust':", 'disabled for static build', color='YELLOW')
         return False
-    if cfg.options.disable_lttng:
-        cfg.msg("Checking for 'lttng-ust':", 'disabled', color='YELLOW')
-        return False
 
     return check_package(cfg, env, 'lttng-ust')
 

wscript

@@ -106,9 +106,9 @@ order to save typing.
         default=False,
         help='Enable benchmarks.')
 
-    g.add_option('--disable-lttng', action='store_true',
+    g.add_option('--enable-lttng', action='store_true',
         default=False,
-        help="Don't use lttng even if supported by board and dependencies available")
+        help="Enable lttng integration")
 
     g.add_option('--disable-libiio', action='store_true',
         default=False,