hash: Use Kconfig to enable hashing in host tools and SPL

At present when building host tools, we force CONFIG_SHAxxx to be enabled
regardless of the board Kconfig setting. This is done in the image.h
header file.

For SPL we currently just assume the algorithm is desired if U-Boot proper
enables it.

Clean this up by adding new Kconfig options to enable hashing on the host,
relying on CONFIG_IS_ENABLED() to deal with the different builds.

Add new SPL Kconfigs for hardware-accelerated hashing, to maintain the
current settings.

This allows us to drop the image.h code and the I_WANT_MD5 hack.

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