kbuild: fix a bug of the u-boot-spl link rule

cmd_u-boot-spl includes $(PLATFORM_LIBS) which changes
when CONFIG_USE_PRIVATE_GCC is updated.  The u-boot-spl image
should be re-linked if any prerequisite is newer than it
or the command line has changed.
$(call, if_changed,...) should be used instead of $(call cmd,...).

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>