image: Drop IMAGE_ENABLE_SHAxxx

We already have a host Kconfig for these SHA options. Use
CONFIG_IS_ENABLED(SHAxxx) directly in the code shared with the host build,
so we can drop the unnecessary indirections.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>