c_headers: add the signess specification

When using:

* sign=0 (We are using unsigned field)
* sign=1 (We are using the most significant bit to give the signess)
* sign=2 (We are using two complements signess)

For example if we have 8bits we will have:

* sign=0 => val=[0,255]
* sign=1 => val=[-127,127]
* sign=2 => val=[-128,127] //because we don't have 0 and -0