Remove #define CONFIG_CRC32

There is no check for CONFIG_CRC32 so the #define in image.h does
nothing. Remove it.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>