kbuild: package: add -e and -u options to some shell 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.

I tweaked mkdebian to cope with optional environment variables.

Remove the explicit "test -n ..." from install-extmod-build.

Both options are described in POSIX. [1]

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

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>