mavlink: cmake generate mavlink headers at build time

- mavlink/mavlink is now directly included as a submodule instead of the generated mavlink/c_library_v2
 - this also switches to mavlink development.xml by default

Tools/setup/requirements.txt

@@ -6,6 +6,7 @@ empy>=3.3
 jinja2>=2.8
 jsonschema
 kconfiglib
+lxml
 matplotlib>=3.0.*
 numpy>=1.13
 nunavut>=1.1.0

Tools/setup/ubuntu.sh

@@ -86,6 +86,8 @@ sudo DEBIAN_FRONTEND=noninteractive apt-get -y --quiet --no-install-recommends i
 	gdb \
 	git \
 	lcov \
+	libxml2-dev \
+	libxml2-utils \
 	make \
 	ninja-build \
 	python3 \