Kconfig: define MD5 dependency for FIT support

FIT images require MD5 support to verify image checksums. So far this
was expressed by defining a CPP symbol in image.h. Since MD5 is now a
first class Kconfig citizen, express that in Kconfig instead.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>