kbuild: fix a bug in regeneration of linker scripts

In some use cases, SPL linker script was not updated even when
it should be.

For instance,

  $ make tricoder_config all
    [ build complete ]
  ... modify include/configs/tricoder.h
  $ make

spl/u-boot-spl.lds should be updated in this case, but it wasn't.

To fix this problem, linker scripts generation should be handled
by $(call if_changed_dep,...) rather than by $(call if_changed,...).

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reported-by: Andreas Bießmann <andreas.devel@googlemail.com>
Tested-by: Andreas Bießmann <andreas.devel@googlemail.com>