sign: handle decode methods sign from can value

sign: handle decode methods sign from can value

- add enumeration that represents the way of encoding:
    enum sign_t
    {
		UNSIGNED = 0,
		SIGN_BIT = 1,
		ONES_COMPLEMENT = 2,
		TWOS_COMPLEMENT = 3,
		SIGN_BIT_EXTERN = 4
    };

- the function handle_sign allows to transform value if negative and
return the sign of the value in order to be multiplied at the end ot the
parcing

Bug-AGL: SPEC-3021

Change-Id: I5766a286488170d930422474b4c4f3a8578ca726
Signed-off-by: Clément Bénier <clement.benier@iot.bzh>
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>