image: introduce genimg_get_kernel_addr()

Kernel address is normally stored as a string argument of bootm or bootz.
This function is taken out from boot_get_kernel() of bootm.c, which can be
reused by others.

Signed-off-by: Bryan Wu <pengw@nvidia.com>
[trini: Fix warnings with CONFIG_FIT]
Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/include/image.h b/include/image.h
index 3e8f78d..ca2fe86 100644
--- a/include/image.h
+++ b/include/image.h
@@ -424,6 +424,7 @@
 #define IMAGE_FORMAT_FIT	0x02	/* new, libfdt based format */
 #define IMAGE_FORMAT_ANDROID	0x03	/* Android boot image */
 
+ulong genimg_get_kernel_addr(char * const img_addr);
 int genimg_get_format(const void *img_addr);
 int genimg_has_config(bootm_headers_t *images);
 ulong genimg_get_image(ulong img_addr);