[os_xml] update makefile
[os_xml] do not define XML_VAR by global compiler flag
[os_xml] make local functions static
[os_xml] remove unused function checkmemory()
[os_xml] make usage of keyword const
[os_xml] fixing compiler warnings
[tests] remove unneccessary maketarget
[tests] fix makefile for clang
[os_xml] replac shared.h with standard headers
[os_xml] remove dead code and unused macros
[os_xml] restructure headers: os_xml.h + os_xml_writer.h -> os_xml.h + os_xml_internal.h
[os_xml] remove unused global line variable from os_write_xml.c and reduce os_xml.c ones to local scope
[os_xml] fix some cppcheck warnings
[os_xml] fix some conversion warnings
[os_xml] remove unused struct element
[os_xml] fix more conversion warnings
[os_xml] comment out dead code