kconfig: add -e and -u options to *conf-cfg.sh scripts

Set -e to make these scripts fail on the first error.

Set -u because these scripts are invoked by Makefile, and do not work
properly without necessary variables defined.

Both options are described in POSIX. [1]

[1]: https://pubs.opengroup.org/onlinepubs/009604499/utilities/set.html

Signed-off-by: Masahiro Yamada <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>

scripts/kconfig/gconf-cfg.sh

···
 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 
+set -eu
+
 cflags=$1
 libs=$2
 

scripts/kconfig/mconf-cfg.sh

···
 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 
+set -eu
+
 cflags=$1
 libs=$2
 

scripts/kconfig/nconf-cfg.sh

···
 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 
+set -eu
+
 cflags=$1
 libs=$2
 

scripts/kconfig/qconf-cfg.sh

···
 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 
+set -eu
+
 cflags=$1
 libs=$2
 bin=$3