Kconfig: Rename SPL_MD5_SUPPORT to SPL_MD5

Drop the _SUPPORT suffix so we can use CONFIG_IS_ENABLED() with this
option.

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>