Kconfig: introduce md5sum command selection

So far CONFIG_MD5SUM would need to be set by a board's include file.
Since the command is really generic, move it over to Kconfig to allow
it to be defined by either a board's defconfig, menuconfig or some
config snippet merged via mergeconfig.sh.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>