Makefile: doesn't need check stack size when dtb is not built

The commit 5fed97af ("Makefile: ensure DTB doesn't overflow into
initial stack") adds an extra check for stack size in BSS if
CONFIG_SYS_INIT_SP_BSS_OFFSET is enabled.
This check, however, doesn't make sense under the configuration where
control dtb won't be built in and it should be void in such cases.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Fixes: 5fed97af ("Makefile: ensure DTB doesn't overflow into initial stack")
Reviewed-by: Stephen Warren <swarren@nvidia.com>