image.h: Tighten up content using handy CONFIG_IS_ENABLED() macro.

In order for CONFIG_IS_ENABLED(FOO) to work we need to move the changes
that CONFIG_FIT_DISABLE_SHA256 makes to be prior to the evaluation by
CONFIG_IS_ENABLED(foo)

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
[trini: Move CONFIG_FIT_DISABLE_SHA256 parts to fix build breakage]
Signed-off-by: Tom Rini <trini@konsulko.com>