EKF: allow initialising without baro depending on configuration

[dagar]

Attempted followup to #931.

• every potential height source maintains a filtered value at the delayed time horizon
• in Ekf::initialiseFilter() only the configured height source is used, but valid height data is required at init time
• height resets use filtered delayed time horizon data rather than most recent measurement

[priseborough]

Can we do a test where the board is started without the baro driver?

[priseborough]

There's a lot of code here for the ability to initialise height without a baro. Can we split this into one PR into what is required for initialisation without baro and a second PR with other improvements, eg median filtering. For example we could initialise height to a fake measurement of zero whilst awaiting the selected height sensor to pass checks.