kconfig: convert Kconfig helper script into a shell script

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 more 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 <[email protected]>
Suggested-by: Igor Grinberg <[email protected]>
Tested-by: Igor Grinberg <[email protected]>
Acked-by: Simon Glass <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Jeroen Hofstee <[email protected]>
Cc: Stephen Warren <[email protected]>