Makefile: rm u-boot.cfg dependencies are missing

Makefile: rm u-boot.cfg dependencies are missing

Prior to the previous patch, a freshly created .u-boot.cfg.cmd may not
correctly represent all dependencies for u-boot.cfg. The previous change
only solved this issue for fresh builds; when performing an incremental
build, the deficient .u-boot.cfg.cmd is already present, so u-boot.cfg
is not rebuilt, and hence .u-boot.cfg.cmd is not rebuilt with the correct
content.

Solve this by explicitly detecting when the dependency file .u-boot.cfg.d
has not been integrated into .u-boot.cfg.cmd, and force u-boot.cfg to be
rebuilt in this case by deleting it first. This is possible since
if_changed_dep will always delete .u-boot.cfg.d when it executes
successfully, so its presence means either that the previous build was
made by a source tree that contained a Makefile that didn't include the
previous patch, or that the build failed part way through executing
if_changed_dep for u-boot.cfg. Forcing a rebuild of u-boot.cfg is required
in the former case, and will cause no additional work in the latter case,
since the file would be rebuilt anyway for the same reason it was being
rebuilt by the previous build.

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