Make blended backend a real backend

[peterbarker]

Tested in SITL.

Board               AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
Durandal                       352    *           184     304               328    352    360
Hitec-Airspeed      *                                                                     
KakuteH7-bdshot                480    *           496     472               656    472    472
MatekF405                      56     *           128     128               -96    80     104
Pixhawk1-1M-bdshot             96                 120     104               64     104    104
f103-QiotekPeriph   *                                                                     
f303-Universal      *                                                                     
iomcu                                                           *                         
revo-mini                      64     *           -96     8                 16     0      104
skyviper-v2450                                    *                                       

[amilcarlucas]

Very cool changes!

[tridge]

we will do some SpringValley flight testing

[peterbarker]

Flight testing at SV went well today.

[tridge]

how does this interact with EKF3 affinity?

[tridge]

can we remove GPS_MAX_RECEIVERS now?

[peterbarker]

Not yet. There are all sorts of interesting decisions to be made revolving around it.

Many of them will end up turning into a :-style iteration across the instances array.

[tridge]

I'd like to understand how this interacts with affinity

[amilcarlucas]

Is this planned for 4.6?

[peterbarker]

I've flown a real copter with blended a bunch now - even with different GPS types it copes.

I've added an autotest that tests blended/affinity before and after.

The logs on master show that when affinity is in play the third core will use the blended GPS.

[Ryanf55]

I tested this in SITL a while back using some of the SIM* params to muck with the GPS health of GPS 1 and 2 and could not get the vehicle to have any serious issues with blended. I was wanting triple-GPS support, and was interested in this.
I haven't tested the latest version.

The new test is nice to ensure GPS errors get blended out.