vfs_i2c: provide a VFS plugin that allows access to the I2C bus

The aim of this is to replace the `I2c` session with a VFS plugin.

My current idea is to have a platform independent library (`lib/vfs_i2c`) and for each controller type a platform dependent VFS plugin (e.g. `vfs_i2c-imx` for the iMX SOCs).

This is still work in progress.