image: Drop IMAGE_ENABLE_SHA1

We already have a host Kconfig for SHA1. Use CONFIG_IS_ENABLED(SHA1)
directly in the code shared with the host build, so we can drop the
unnecessary indirection.

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>