scripts: kconfig: merge_config.sh: pass output file as awk variable

scripts: kconfig: merge_config.sh: pass output file as awk variable

The refactoring commit 5fa9b82cbcfc5 ("scripts: kconfig:
merge_config.sh: refactor from shell/sed/grep to awk") passes
$TMP_FILE.new as ARGV[3] to awk, using it as both an output destination
and an input file argument. When the base file is empty, nothing is
written to ARGV[3] during processing, so awk fails trying to open it
for reading:

    awk: cmd. line:52: fatal: cannot open file
    `./.tmp.config.grcQin34jb.new' for reading: No such file or directory
    mv: cannot stat './.tmp.config.grcQin34jb.new': No such file or directory

Pass the output path via -v outfile instead and drop the FILENAME ==
ARGV[3] { nextfile }.

Fixes: 5fa9b82cbcfc5 ("scripts: kconfig: merge_config.sh: refactor from shell/sed/grep to awk")
Signed-off-by: Daniel Gomez <da.gomez@samsung.com>
Link: https://patch.msgid.link/20260310-fixes-merge-config-v1-1-beaeeaded6bd@samsung.com
Signed-off-by: Nathan Chancellor <nathan@kernel.org>