kbuild: refactor more IMX image rules

This commit avoids generating ./SPL twice.
  - Fist time descending to spl/
  - Second time as a prerequisite of u-boot-with-spl.imx,
    u-boot-with-nand-spl.imx.

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