Create system state entry in dataman - ATTN Anton

Create persistent system state id for data manager to store system state
that will persist across resets.
dataman: allow writing empty items with nullptr pointer to data
mavlink: waypoint manager fixes, mission saving on reboot
mavlink: retry timeout moved to define
Merge branch 'master' into dataman_state
Merge branch 'mavlink_stack' into dataman_state
dataman: rename SYSTEM_STATE section to MISSION_STATE
Merge branch 'navigator_rewrite' into dataman_state_nav_rewrite
Merge branch 'navigator_rewrite' into dataman_state_nav_rewrite
Move MISSION_STATE read/write from mavlink to navigator and commander
mavlink: fix WPM initialization
mavlink: mission manager moved to separate class and reworked
Merge branch 'navigator_rewrite' into dataman_state_nav_rewrite
mavlink: don't lock dataman when updating mission state
Merge branch 'navigator_rewrite' into dataman_state_nav_rewrite
Merged master
Fix failed merge
Merge branch 'master' into dataman_state_nav_rewrite
mavlink, commander: bugs fixed
rcS: start dataman before commander