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
Acked-by: Simon Glass