Makefile: copy SPL_FIT_SOURCE in build directory

Copy the .its source file selected by CONFIG_SPL_FIT_SOURCE
in builddir and in a file named "u-boot.its".

This patch avoid compilation issue when CONFIG_SPL_FIT_SOURCE is used
and KBUILD_OUTPUT is defined, in buildman for example.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Tested-by: Marek Vasut <marex@denx.de>
Acked-by: Marek Vasut <marex@denx.de>
diff --git a/Makefile b/Makefile
index b8a4b50..863ddd9 100644
--- a/Makefile
+++ b/Makefile
@@ -1322,7 +1322,9 @@
 # Boards with more complex image requirements can provide an .its source file
 # or a generator script
 ifneq ($(CONFIG_SPL_FIT_SOURCE),"")
-U_BOOT_ITS = $(subst ",,$(CONFIG_SPL_FIT_SOURCE))
+U_BOOT_ITS := u-boot.its
+$(U_BOOT_ITS): $(subst ",,$(CONFIG_SPL_FIT_SOURCE))
+	$(call if_changed,copy)
 else
 ifneq ($(CONFIG_SPL_FIT_GENERATOR),"")
 U_BOOT_ITS := u-boot.its