Support parallel flash vendor extensions for top/bottom boot sectors.

[docbacardi]

Testing Spansion S29AL and S29GL devices showed an "interesting" detail in the CFI table: Both devices are available with top and bottom boot sectors. This is basically a bunch of smaller sectors at the start or at the end of the flash. Unfortunately the top and bottom versions have the same erase block info in the CFI table, where the small sectors are always at the start of the flash.
There is an entry in the primary vendor specific extensions showing if the device is a top or a bottom boot sector device. This could be used to swap the entries in the erase block info.