uboot-imx

Makefile: Use Kbuild style for system_map.o generation step

The command generating the "common/system_map.o" file was always shown
during the build making the output messy. Now it is called using the
Kbuild "cmd" macro, so that the full command is shown only when
building in verbose mode.

Signed-off-by: Vasili Galka <vvv444@gmail.com>