Fix defines needed to enable command sha1sum

Documented is CONFIG_CMD_SHA1, through confusion in the source
CONFIG_CMD_SHA1 and CONFIG_CMD_SHA1SUM has to be used to enable
sha1sum.

Fix both, the documentation and the source, so that only
CONFIG_CMD_SHA1SUM is needed to enable the command sha1sum.

Signed-off-by: Alexander Holler <holler@ahsoftware.de>
diff --git a/README b/README
index 8e1a9ff..755d17c 100644
--- a/README
+++ b/README
@@ -675,7 +675,7 @@
 					  (requires CONFIG_CMD_I2C)
 		CONFIG_CMD_SETGETDCR	  Support for DCR Register access
 					  (4xx only)
-		CONFIG_CMD_SHA1		  print sha1 memory digest
+		CONFIG_CMD_SHA1SUM	  print sha1 memory digest
 					  (requires CONFIG_CMD_MEMORY)
 		CONFIG_CMD_SOURCE	  "source" command Support
 		CONFIG_CMD_SPI		* SPI serial bus support