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>