uboot-imx

cmd: remove CONFIG_SOURCE support in Makefile

This line is no more needed and can be removed.

Only CONFIG_CMD_SOURCE is defined in Kconfig and
used in defconfig files.

CONFIG_SOURCE if not defined in source code and
"config SOURCE" is not present in any Kconfig.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>