debian/bin/gencontrol.py: Fix handling of debug info switches

Setting $DEBIAN_KERNEL_DISABLE_DEBUG or clearing the [build]debug-info
config variable should disable generation of debug info and associated
packages.  However the recent refactoring of the handling of such
environment variables resulted in different switches being obeyed in
different places.

Set the build_debug variable based on both switches and then use that
consistently.

(cherry picked from commit 36f061ce80471a0203c174c4e241b70af913c260)

debian/bin/gencontrol.py

@@ -496,9 +496,12 @@ class Gencontrol(Base):
         if config_entry_build.get('vdso', False):
             makeflags['VDSO'] = True
 
-        build_debug = config_entry_build.get('debug-info')
-
         if not self.disable_debug:
+            build_debug = config_entry_build.get('debug-info')
+        else:
+            build_debug = False
+
+        if build_debug:
             makeflags['DEBUG'] = True
             packages_own.extend(self.process_packages(
                 self.templates['control.image-dbg'], vars))