Since commit a26cd04920 ("arch: Make board selection choices
optional"), Kconfig could create such an insane .config file that
no SoC/board is selected.
This is now a real problem for Buildroot, for example.
(http://lists.busybox.net/pipermail/buildroot/2015-July/135125.html)
This commit drops the "optional" from the ARM target select menu
in favor of "Versatile family".
Rationale:
- Historically, Linux chose versatile_defconfig as the default
of ARM defconfig. (arch/arm/Makefile of Linux describes:
KBUILD_DEFCONFIG := versatile_defconfig)
- It was published by ARM Ltd.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>