events: Move implementation of events::send() to lib/events

Events have a global, system-wide sequence number, which must be handled
atomically, (fetching and incrementing the sequence AND sending the event
to uORB must be atomic). Currently in FLAT mode, only one instance of this
sequence number exists, so it is OK to have it in px4_platform.

However, in PROTECTED mode px4_platform is instantiated both in kernel-
and user spaces, which makes two instances of this sequence number, which
causes problems in the mavlink event handling logic.

When mavlink receives and handles events, it expects that:
- The sequence numbers arrive in order (seq n is followed by n+1 etc)
- It increments by 1
- There is only one instance of the sequence number

In PROTECTED mode this is violated, as the kernel and user sequence
numbers run freely on their own. This patch fixes the issue by moving
the event backend to the kernel and by providing user access to it via
ioctl.

CMakeLists.txt

@@ -414,6 +414,8 @@ endif()
 #
 add_library(parameters_interface INTERFACE)
 add_library(kernel_parameters_interface INTERFACE)
+add_library(events_interface INTERFACE)
+add_library(kernel_events_interface INTERFACE)
 
 include(px4_add_library)
 add_subdirectory(src/lib EXCLUDE_FROM_ALL)
@@ -440,8 +442,11 @@ add_subdirectory(src/lib/parameters EXCLUDE_FROM_ALL)
 if(${PX4_PLATFORM} STREQUAL "nuttx" AND NOT CONFIG_BUILD_FLAT)
 	target_link_libraries(parameters_interface INTERFACE usr_parameters)
 	target_link_libraries(kernel_parameters_interface INTERFACE parameters)
+	target_link_libraries(events_interface INTERFACE usr_events)
+	target_link_libraries(kernel_events_interface INTERFACE events)
 else()
 	target_link_libraries(parameters_interface INTERFACE parameters)
+	target_link_libraries(events_interface INTERFACE events)
 endif()
 
 # firmware added last to generate the builtin for included modules