POSIX: Detailed design and implementation of POSIX file interface

[alvieboy]

Detailed design and implementation of POSIX file interface.

• Support commands for POSIX file operations in spectcmd.c
• Support RST$08 interface for Spectrum interfacing.

This will need a file handle interface to separate ESP32 handles from Spectrum handles. To be understood if we also need some sort of driver infrastructure.

API should implement equivalent methods for the following POSIX functions:

• getcwd
• chdir
• open
• close
• read
• write
• opendir
• readdir
• closedir
• ioctl
• fcntl

The directory handles are still TBD, because we need to map those to the internal ESP32 directory handles as well.
The DEVELOPMENT document shall be updated to include detailed information about the command API.
The SPECTRUMAPI document shall be updated to include the RST$08 API.