build: added APM_BUILD_DIRECTORY define to build

this makes it possible to have different default parameter values for
different vehicle types in libraries, as long as you use the makefile
based build
AP_Vehicle: added defines for main vehicle types
AP_NavEKF: added a mechanism for per-vehicle-type defaults in EKF
AP_Vehicle: added APM_BUILD_TYPE() macro

this makes checking build type less fragile
AP_NavEKF: use APM_BUILD_TYPE() macro
RC_Channel: prevent a numerical exception with bad mount parameters
AP_NavEKF : Reduce GPS position innovation consistency fail threshold

This patch reduces the maximum acceptable GPS jump from approximately 16 to 8 metres
This will provide copters with more protection for close in loiter situations
AP_NavEKF : Reduce Magnetometer innovation consistency check threshold