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

[pussuw]

Solved Problem

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.

Solution

Move events::send() to kernel in PROTECTED build ensuring only a single event sequence number exists in the system

Alternatives

Do the event dispatch logic in some other manner, open to suggestions. This does add a lot of boiler plate code for something extremely simple, i.e. get seq, increment it, set seq to event and send via uorb.

[bkueng]

We could partly use the uorb generation, but this is still a bit better.