Kconfig: Write tmp files into same directory as target files

Kconfig: Write tmp files into same directory as target files

This removes the need for COREBOOT_BUILD_DIR in Kconfig. Since the
original files will be replaced with the tmp file, the parent directory
already needs to be writable.

Before this change, the tmp files would be created in the CWD (src) if
COREBOOT_BUILD_DIR was not specified.

BUG=b:112267918
TEST=emerge-grunt coreboot and verified no tmp files were created in the
src directory.

Change-Id: Icdaf2ff3dd1ec98813b75ef55b96e38e1ca19ec7
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34244
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>