tools: kwbimage: Fix arithmetic with void pointers

tools: kwbimage: Fix arithmetic with void pointers

Arithmetic with void pointers, e.g. a - b where both a and b are void
pointers, is undefined in the C standard. Since we are operating with
byte data here, we switch the void pointers to uint8_t pointers, and add
the necessary casts.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>