Do this by indicating the end of the appropriate regions of memory.

Note that MAX_PDA_SIZE should only apply to the PDA block read from
flash/EEPROM, and has been erronously applied to the pdr elements.
Remove the macro, and use the actual PDA size passed down by the caller.

We also fix up some of the types used, marking as much as possible
const, and using void* for the end pointers.
Signed-off-by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>