WIP: Embedded FFS support

[thomafred]

Description

The aim of this pull request is to provide an interface for generic file systems such as SPIFFS, YAFFS, etc for platforms without POSIX file IO support.

As a caveat, CMake support will also be added.

Checklist

[x] Add support for CMake build system.
[ ] Rudimentary housekeeping

Comments

In order to build:

$ cd ${PROJECT_ROOT}
$ mkdir -p build && cd build
$ cmake ..
$ make
$ make test

make test is just a placeholder. Implement tests in test_microtar.c.

[thomafred]

Edit: Updated pull request scope and added WIP-tag.