gfxboard: rewind Picasso IV flash before autoconfig

gfxboard: rewind Picasso IV flash before autoconfig

Picasso IV autoconfig is read from the flash image, but the
initializer can run more than once while the expansion list is
scanned and sorted. loadp4rom() leaves the flash zfile positioned
later in the image, so a later dry scan read all-0xff data and the
board looked like it had no autoconfig block.

Seek back to the beginning before reading the Picasso IV
autoconfig bytes so repeated scans see the same board data.