r.blend use standardized options (https://trac.osgeo.org/grass/ticket/2409)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@62989 15284696-431f-4ddb-bdfa-cd5b030d7da7

scripts/r.blend/r.blend.py

@@ -27,12 +27,8 @@
 #% key: second
 #% description: Name of second raster map for blending
 #%end
-#%option 
-#% key: output_prefix
-#% type: string
-#% description: Prefix for red, green and blue output raster maps containing
-#% key_desc : name
-#% required : yes
+#%option G_OPT_R_BASENAME_OUTPUT
+#% description: Basename for red, green and blue output raster maps
 #%end
 #%option
 #% key: percent