odroid: Fix target platform compilation issue on Odroid-XU3

odroid: Fix target platform compilation issue on Odroid-XU3

Resolved a compilation issue where the build system attempted
to compile for the Odroid-XU3 platform instead of the specified
Exynos4412 platform due to an incorrect CONFIG_SYS_BOARD setting.
Updated the Makefile to ensure that compilation only occurs for
the Odroid-XU3 specified target platform.

Now, object files will be generated only when building for the
TARGET_ODROID platform.

Signed-off-by: Anand Moon <linux.amoon@gmail.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>