Create px4_fmu-v5-secure_default make target for applying stig compliance

[jrn90]

A custom make target (specifically the fmu-v5-secure) will allow us to control over a few things:

• Nuttx kernel configuration
• Ability to customize the bootloader
• Set initialization parameters for board modules
• Start / Stop applications / modules
• Reference for other make targets to build against (code coverage, static analysis, etc)

The _default portion of the make target is the board configuration. This does a few things like setting platform (nuttx or linux), architecture (ARM or x86), drivers (gps, battery sensor, camera), modules, etc...

• [x] Follow Board Porting Guide to basline fmu-v5-secure from fmu-v5
• [ ] Update documentation / wiki with understanding of files, configuration settings, additional findings, etc...