Makefile: Do not call useless command 'true'

Makefile: Do not call useless command 'true'

Macro 'cmd_objcopy_uboot' currently does not work with passed empty command
expanded from 'cmd_static_rela' and therefore dummy command 'true' is set
in 'cmd_static_rela' to workaround this issue.

Eliminate it now by fixing 'cmd_objcopy_uboot' macro to work also with
empty 'cmd_static_rela' macro and remove useless invocation of command
'true'.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>