I found something by Googling about roots and negative meaning but more than it I want to know if there is a positive or neutral usage too. Is it something like geek or nerd and the other slangy stereotypes or a serious history is hidden behind ?
The short: yes, there is "history" behind the term similar to the information from the OED via @FumbleFingers. However, many white farmers were and are proud of their hard work ethic and are not ashamed of a farmer's tan.
The degree of pejoration, however, is much more complex, and I'm just baffled at the strong responses that the use of redneck is always negative. The long: a different perspective.
In the last 20+ years of country and country pop music, lyricists have ?reclaimed redneck much like the Black community has re-purposed nigger. The 2004 country hit "Redneck Woman" by Gretchen Wilson exemplifies the "I'm X and proud of it" sentiment, @JonHanna's excellent response.
*In subsequent refrains, she names Tanya Tucker and ol' Bocephus
Her bio includes a National Coalition for Literacy Leadership Award in 2009, the same year she launched her own record label, "Redneck Records."
Similar examples of redneck ?pride can be heard in the following examples, which is by no means a comprehensive list:
n.b.: The main point expressed in most of these songs is not self-denigrating humor à la Jeff Foxworthy or Chris Rock. Country music more often expresses praise and positive identification with its heritage.
Personally, I'm a Virginia native, and while it's more likely you'd call me a nerd than a redneck, I would not be offended by either. In my experience, Southerners are perfectly likely to identify themselves with the term.
One of my favorite country songs is Sammy Kershaw's Queen of my Double Wide Trailer, with this refrain:
Definitely an example of a red neck being a desirable quality.
Sometimes people can playfully refer to themselves as "rednecks", but almost always the term is considered a derogatory one and synonymous with "white trash," "bigotry," or overly "provincial" folks. In the U.S. rednecks are thought to come from small farming communities, especially in the south, or blue collar towns. They, however, equally reside in the large cities. Recently, cable TV has begun to glorify the redneck.
Arguably this is General Reference, but the OED entry for slang redneck (orig. N. Amer.) is...
Obviously the white farm labourer would get a red sunburnt neck that wouldn't be noticeable on black workers (also negatively stereotyped at the time). And opinionated reactionaries, etc. can get quite worked up about innovation and change, becoming red in the face and neck when venting their spleen.
I've used and heard redneck many thousands of times, but never once positively. Always negative.
I know of a context in which "wife-swapping sodomite" had positive connotations: someone had referred to her political opponents as "a bunch of wife-swappers and sodomites", and some of said opponents made a humorous attack on her out of referring to themselves as "wife-swapping sodomites".
Any negative term can be re-purposed.
While some might argue Lenny Bruce came close, generally they cannot be re-purposed by people outside of the group they describe. The re-purposing often requires humour and while that can be light-hearted, often it's with a certain grim irony that doesn't lose sight of the negative sense. While a few can make complete reversals, they have generally already lost some of the negativity already (gay could probably not have become the neutral term it was if many people realised that it was applied to homosexuals because it once had meanings of "prostitute" and "promiscuous").
You can normally find a bunch of positive uses for just about any term by googling for "X and proud of it" or "I'm an X and proud of it", but to understand such positive senses you have to acquaint yourself more with the demographic and culture it covers than you do with uses of the term generally.
Generally pejorative - I would probably not use it unless you were sure of your audience.
But Jeff Foxworthy and the Blue Collar Comedy have reclaimed it in a humourous context - rather like black comics use of nigger.