kconfig: merge_config: create temp files in OUTPUT

merge_config.sh crates temporary files in the current directory
regardless of the OUTPUT value. This causes build artifacts to pollute
the source tree when using out of tree builds (e.g. using -O or
KCONFIG_CONFIG).

This creates temporary files in the OUTPUT directory instead of
hardcoding it.

Signed-off-by: Daniel Gomez <da.gomez@samsung.com>