lib: Allow MD5 to be enabled in SPL

At present the MD5 option cannot be enabled by board configs since it has
no Kconfig name. It is generally enabled, so long as FIT support is
present. But not all boards use FIT, particularly in SPL

Fix this and add an option for SPL as well. This allows board code to call
md5() even if FIT support is not enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>