doc: signature: update algorithm addition description

U-Boot now uses the U_BOOT_CRYPTO_ALGO() macro.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/uImage.FIT/signature.txt b/doc/uImage.FIT/signature.txt
index bc123f5..b670741 100644
--- a/doc/uImage.FIT/signature.txt
+++ b/doc/uImage.FIT/signature.txt
@@ -56,10 +56,10 @@
 under 5KB on Tegra Seaboard, for example.
 
 It is relatively straightforward to add new algorithms if required. If
-another RSA variant is needed, then it can be added to the table in
-image-sig.c. If another algorithm is needed (such as DSA) then it can be
-placed alongside rsa.c, and its functions added to the table in image-sig.c
-also.
+another RSA variant is needed, then it can be added with the
+U_BOOT_CRYPTO_ALGO() macro. If another algorithm is needed (such as DSA) then
+it can be placed in a directory alongside lib/rsa/, and its functions added
+using U_BOOT_CRYPTO_ALGO().
 
 
 Creating an RSA key pair and certificate