1440, "hard wafer," but the specific application to a thin, crisp biscuit is 1739.
Cracker-barrel (adj.) "emblematic of down-home ways and views" is from 1877.
Cracker, Southern U.S. derogatory term for "poor, white trash" (1766), is from c.1450
crack "to boast" (e.g.
not what it's cracked up to be), originally a Scottish word. Especially of Georgians by 1808, though often extended to residents of northern Florida.
"I should explain to your Lordship what is meant by crackers; a name they have got from being great boasters; they are a lawless set of rascalls on the frontiers of Virginia, Maryland, the Carolinas and Georgia, who often change their places of abode." [1766, G. Cochrane]