image: Tidy up genimg_get_kernel_addr_fit()

This function does not modify its first argument, so mark it const. Also
move the comments to the header file and expand them to provide more
useful information.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/boot/bootm.c b/boot/bootm.c
index 598f880..e323c8b 100644
--- a/boot/bootm.c
+++ b/boot/bootm.c
@@ -122,8 +122,7 @@
 #endif
 	ulong		img_addr;
 	const void *buf;
-	const char	*fit_uname_config = NULL;
-	const char	*fit_uname_kernel = NULL;
+	const char *fit_uname_config = NULL, *fit_uname_kernel = NULL;
 #if CONFIG_IS_ENABLED(FIT)
 	int		os_noffset;
 #endif
diff --git a/boot/image-board.c b/boot/image-board.c
index d500da1..062c76b 100644
--- a/boot/image-board.c
+++ b/boot/image-board.c
@@ -198,22 +198,7 @@
 	}
 }
 
-/**
- * genimg_get_kernel_addr_fit - get the real kernel address and return 2
- *                              FIT strings
- * @img_addr: a string might contain real image address
- * @fit_uname_config: double pointer to a char, will hold pointer to a
- *                    configuration unit name
- * @fit_uname_kernel: double pointer to a char, will hold pointer to a subimage
- *                    name
- *
- * genimg_get_kernel_addr_fit get the real kernel start address from a string
- * which is normally the first argv of bootm/bootz
- *
- * returns:
- *     kernel start address
- */
-ulong genimg_get_kernel_addr_fit(char * const img_addr,
+ulong genimg_get_kernel_addr_fit(const char *const img_addr,
 				 const char **fit_uname_config,
 				 const char **fit_uname_kernel)
 {