Further git submodule improvements

.travis.yml

@@ -93,6 +93,7 @@ env:
     - PX4_AWS_BUCKET=px4-travis
 
 script:
+  - git submodule update --init --recursive
   - make check_format
   - arm-none-eabi-gcc --version
   - echo 'Building POSIX Firmware..' && make posix_sitl_default

Makefile

@@ -109,8 +109,9 @@ endif
 # describe how to build a cmake config
 define cmake-build
 +@if [ $(PX4_CMAKE_GENERATOR) = "Ninja" ] && [ -e $(PWD)/build_$@/Makefile ]; then rm -rf $(PWD)/build_$@; fi
++git submodule init
 +Tools/check_submodules.sh 
-+@if [ ! -e $(PWD)/build_$@/CMakeCache.txt ]; then mkdir -p $(PWD)/build_$@ && cd $(PWD)/build_$@ && cmake .. -G$(PX4_CMAKE_GENERATOR) -DCONFIG=$(1); fi
++@if [ ! -e $(PWD)/build_$@/CMakeCache.txt ]; then git submodule sync && git submodule init && mkdir -p $(PWD)/build_$@ && cd $(PWD)/build_$@ && cmake .. -G$(PX4_CMAKE_GENERATOR) -DCONFIG=$(1); fi
 +$(PX4_MAKE) -C $(PWD)/build_$@ $(PX4_MAKE_ARGS) $(ARGS)
 endef