kconfig: convert Kconfig helper script into a shell script

Commit 51148790 added scripts/multiconfig.py written in Python 2
to adjust Kconfig for U-Boot.

It has been hard for Python 3 users because Python 2 and Python 3
are not compatible with each other.

We are not happy about adding a new host tool dependency
(in this case, Python version dependency) for the core build process.
After some discussion, we decided to use only basic tools.

The script may get a bit unreadable by shell scripting, but we believe
it is worthwhile.

In addition, this commit revives "<board>_config" target that is
equivalent to "<board>_defconfig" for backwards compatibility.
It is annoying to adjust various projects which use U-Boot.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Suggested-by: Igor Grinberg <grinberg@compulab.co.il>
Cc: Tom Rini <trini@ti.com>
Cc: Jeroen Hofstee <jeroen@myspectrum.nl>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stephen Warren <swarren@wwwdotorg.org>