mgmt/mcumgr/lib: Deprecate CONFIG_IMG_MGMT_UL_CHUNK_SIZE

The IMG_MGMT_UL_CHUNK_SIZE is no longer used to control size of
intermediate buffer used for application image chunk processing,
instead directly request trasport buffer is used.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/subsys/mgmt/mcumgr/lib/cmd/img_mgmt/Kconfig b/subsys/mgmt/mcumgr/lib/cmd/img_mgmt/Kconfig
index cf94fc5..c46dfc2 100644
--- a/subsys/mgmt/mcumgr/lib/cmd/img_mgmt/Kconfig
+++ b/subsys/mgmt/mcumgr/lib/cmd/img_mgmt/Kconfig
@@ -13,10 +13,14 @@
 
 if MCUMGR_CMD_IMG_MGMT
 config IMG_MGMT_UL_CHUNK_SIZE
-	int "Maximum chunk size for image uploads"
+	int "Maximum chunk size for image uploads (DEPRECATED)"
 	default 512
 	depends on MCUMGR_CMD_IMG_MGMT
 	help
+	  DEPRECATED: The mcumgr no longer copies chunk of file into intermediate
+	  buffer, instead the request transport buffer is used directly to
+	  process image chunk.
+
 	  Limits the maximum chunk size for image uploads, in bytes.  A buffer of
 	  this size gets allocated on the stack during handling of a image upload
 	  command.