Makefile: Fix u-boot-nodtb.bin target

This change fixes two issues when building u-boot-nodtb.bin target:

* Remove intermediate binary u-boot-nodtb.bin from disk when static_rela
  call (which modifies u-boot-nodtb.bin binary) failed. It is required
  because previous objcopy call creates binary and static_rela finish it.

* Do not call static_rela cmd when u-boot-nodtb.bin binary was not
  created/updated by previous objcopy call.

Second fix would ensure that u-boot-nodtb.bin binary is not updated when
all prerequisites were up-to-date. And therefore final binary u-boot.bin
is not updated in case all prerequisites were not modified and were
up-to-date.

Now running 'make SOURCE_DATE_EPOCH=0 u-boot.bin' second time now does not
touch u-boot.bin binary in case nothing was modified, so GNU make can
correctly detect that everything is up-to-date.

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