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>
diff --git a/lib/Kconfig b/lib/Kconfig
index b16062f..65c0157 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -101,6 +101,10 @@
 	  SHA1/SHA256 progressive hashing.
 	  Data can be streamed in a block at a time and the hashing
 	  is performed in hardware.
+
+config MD5
+	bool
+
 endmenu
 
 menu "Compression Support"