build: fix symlinked .config handling

When running "make menuconfig" with symlinked .config (e.g. to
env/.config) it renames symlink to .config.old, creates new .config file
and writes updated configuration here.

This breaks the desired workflow when changes in the configuration could
be checked using "scripts/env diff" and commited with
"scripts/env save". Since the env/.config file is not updated.

Fix this issue by exporting KCONFIG_OVERWRITECONFIG=1, which forces
mconf to overwrite the .config content, instead of renaming it and
creating a new file. This variable is set only if .config is a symlink,
otherwise the variable is not exported and the old behaviour is
preserved.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>