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