Makefile: use if_change_dep for u-boot.cfg

cmd_cpp_cfg generates a dependency output, but because it's invoked using
if_changed rather than if_changed_dep, that dependency file is ignored.
This results in Kbuild not knowing about which files u-boot.cfg depends
on, so it may not be rebuilt when required.

A practical result of this is that u-boot.cfg may continue to reference
CONFIG_ options that no longer exist in the source tree, and this can
cause the adhoc config options check to fail.

This change modifies Makefile to use if_changed_dep, which in turn causes
all dependencies to be known to the next make invocation.

Signed-off-by: Stephen Warren <swarren@nvidia.com>