Changes for local relative includes to work

[jpmeijers]

Arduino allows one to have a directory structure as follows:

sketch/
- sketch.ino
+ src/
  + SdFat/
    - library.properties
    - LICENSE.md
    - README.md
    + src/
      - SdFat.h
      + FatLib
      ...

This is useful when one want to commit specific versions of libraries into git as submodules. This is also useful in cases where you do not trust what is on the system, or if you want to save the effort of installing many libraries.

To get SdFat to compile in this circumstance, I had to make a few changes to the relative includes.

References:
arduino/arduino-builder#148
arduino/Arduino#5186 (comment)

[greiman]

I am not making mods to SdFat V1. I never apply pulls because they often don't work for other systems like Particle and other IDEs.

I will look at this for SdFat V2.