mkimage: Improve documentation of algo-name parameter

Addresses the feedback provided on 5902a397d029 ("mkimage: Allow to
specify the signature algorithm on the command line") which raced with
the merge.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/mkimage.1 b/doc/mkimage.1
index 0734bd3..fc84cca 100644
--- a/doc/mkimage.1
+++ b/doc/mkimage.1
@@ -158,7 +158,7 @@
 .TP
 .BI "\-o [" "signing algorithm" "]"
 Specifies the algorithm to be used for signing a FIT image. The default is
-taken from the target signature nodes 'algo' properties.
+taken from the signature node's 'algo' property.
 
 .TP
 .BI "\-p [" "external position" "]"
diff --git a/tools/imagetool.h b/tools/imagetool.h
index a410551..c3f80fc 100644
--- a/tools/imagetool.h
+++ b/tools/imagetool.h
@@ -71,7 +71,9 @@
 	const char *keydest;	/* Destination .dtb for public key */
 	const char *keyfile;	/* Filename of private or public key */
 	const char *comment;	/* Comment to add to signature node */
-	const char *algo_name;	/* Algorithm name to use hashing/signing */
+	/* Algorithm name to use for hashing/signing or NULL to use the one
+	 * specified in the its */
+	const char *algo_name;
 	int require_keys;	/* 1 to mark signing keys as 'required' */
 	int file_size;		/* Total size of output file */
 	int orig_file_size;	/* Original size for file before padding */