kconfig: remove redundant "string" type in arch and board Kconfigs

Now the types of CONFIG_SYS_{ARCH, CPU, SOC, VENDOR, BOARD, CONFIG_NAME}
are specified in arch/Kconfig.

We can delete the ones in arch and board Kconfig files.

This commit can be easily reproduced by the following command:

find . -name Kconfig -a ! -path ./arch/Kconfig | xargs sed -i -e '
/config[[:space:]]SYS_\(ARCH\|CPU\|SOC\|\VENDOR\|BOARD\|CONFIG_NAME\)/ {
N
s/\n[[:space:]]*string//
}
'

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

kconfig: add board Kconfig and defconfig files

This commit adds:
- arch/${ARCH}/Kconfig
provide a menu to select target boards
- board/${VENDOR}/${BOARD}/Kconfig or board/${BOARD}/Kconfig
set CONFIG macros to the appropriate values for each board
- configs/${TARGET_BOARD}_defconfig
default setting of each board

(This commit was automatically generated by a conversion script
based on boards.cfg)

In Linux Kernel, defconfig files are located under
arch/${ARCH}/configs/ directory.
It works in Linux Kernel since ARCH is always given from the
command line for cross compile.

But in U-Boot, ARCH is not given from the command line.
Which means we cannot know ARCH until the board configuration is done.
That is why all the "*_defconfig" files should be gathered into a
single directory ./configs/.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>