Makefile: Improve generated_defconfig file handling

Commit 2027e99e61aa ("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: 2027e99e61aa ("Makefile: Run defconfig files through the C preprocessor")
Acked-by: Andrew Davis <afd@ti.com>
diff --git a/.gitignore b/.gitignore
index 3301481..d9a64d7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -63,6 +63,7 @@
 /spl/
 /tpl/
 /defconfig
+/generated_defconfig
 
 #
 # Generated include files