Copter: Use RTL_CLIMB_MIN in cone slope.

[lthall]

This PR replaces the hard coded height increase in the cone slope calculation with the parameter RTL_CLIMB_MIN.

The current behaviour determines the RTL altitude based on the minimum of:

• The parameter RTL_ALT (default 10m)
• The hash define RTL_ALT_MIN (2m)
• The current altitude + RTL_CLIMB_MIN

If we are in the cone slope we can reduce the altitude to a minimum of:

• rtl_return_dist_cm * RTL_CONE_SLOPE
• The current altitude + RTL_ABS_MIN_CLIMB (2.5m)

This means that even if you are right above the RTL location the aircraft will increase in altitude by 2.5m before descending.

This PR changes the minimum altitude in the cone to:

• rtl_return_dist_cm * RTL_CONE_SLOPE
• MAX(RTL_ALT_MIN, The current altitude + RTL_CLIMB_MIN)

This means that when you are right over the landing point the aircraft will only increase in altitude by RTL_CLIMB_MIN or to a minimum of 2m.

[rmackay9]

RTL_CLIMB_MIN defaults to zero so this will change the behaviour for most users. Maybe you can write a couple of lines explaining what the difference will be from a user's point of view?

[IamPete1]

Looks good.