fpu with single/double precision - removing redundancy

- removing redundancy as discussed in PR #2087:
 - in target.json the core option can have only this values : "Cortex-M0", "Cortex-M0+", "Cortex-M1", "Cortex-M3", "Cortex-M4", "Cortex-M7", "Cortex-A9" - Cortex-M4F and Cortex-M7F removed
 - in target.json an additional fpu option with values: "single" and "double" can be used
- build and export scripts are changed to handle this

- tested (compiling, running on hardware) with nucleo_f767 (cortex-m7 with double precision fpu), nucleo_f746 (cortex-m7 with single precision fpu), nucleo_f446 and nucleo_l467 (cortex-m4 with single precision fpu), teensy31 (cortex-m4 without fpu - only build test), nucleo_l073 (cortex-m0)
- singletest results are added to PR #2087 comments

Author: ohagendorf