Makefile: Improve generated_defconfig file handling

Commit 2027e99e ("Makefile: Run defconfig files through the C
preprocessor") adds `generated_defconfig' file, but fails to clean that
up. It might be useful to have that file around after `make' is done,
but it's better to clean that up on `make clean'. Also we probably want
to hide it in `git status' list. This patch makes the described changes,
and also adds `-P' parameter to the CPP command that produces the
`generated_defconfig' to avoid generating linemarkers.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Fixes: 2027e99e ("Makefile: Run defconfig files through the C preprocessor")
Acked-by: Andrew Davis <afd@ti.com>