mach-imx: do not use if_changed more than once per target

doc/develop/makefiles.rst has the following note:

  if_changed should not be used more than once per target.
  It stores the executed command in a corresponding .cmd
  file and multiple calls would result in overwrites and
  unwanted results when the target is up to date and only the
  tests on changed commands trigger execution of commands.

The mach-imx Makefile does not follow this recommandation, so fix it
by implementing a single command that performs both the cpp_cfg
and imx9_check actions.

This change fixes an issue with "tools/buildman/buildman imx8ulp_evk"
failing every other time [1].

[1] https://lists.denx.de/pipermail/u-boot/2024-August/563192.html



Fixes: f637dfe8 ("mach-imx: Add i.MX93 binman support.")
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>