A recent change added a warning when yamllint is not installed, as it is
needed for 'make dt_binding_check'. However, it also changed
DT_SCHEMA_LINT to be a simple make variable, which is evaluated when a
Makefile is evaluated. This causes a warning when running 'make clean',
as Documentation/devicetree/bindings/Makefile has a "clean-files"
variable:

  $ make -s clean
  which: no yamllint in (...)
  warning: python package 'yamllint' not installed, skipping

Make DT_SCHEMA_LINT a recursive variable so it is evaluated only when it
is used. The warning still triggers when 'make dt_binding_check' is run.

Fixes: b3e664a7 ("dt-bindings: kbuild: Print a warning if yamllint is not found")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220315202542.2071351-1-nathan@kernel.org