ARM: drop "optional" from target select in favor of ARCH_VERSATILE

Since commit a26cd049 ("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: NMasahiro Yamada <yamada.masahiro@socionext.com>

arch/arm/Kconfig

@@ -64,7 +64,7 @@ config SEMIHOSTING
 
 choice
 	prompt "Target select"
-	optional
+	default ARCH_VERSATILE
 
 config ARCH_AT91
 	bool "Atmel AT91"

configs/versatileab_defconfig

 CONFIG_ARM=y
-CONFIG_ARCH_VERSATILE=y
 CONFIG_SYS_EXTRA_OPTIONS="ARCH_VERSATILE_AB"
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_BOOTD is not set

configs/versatilepb_defconfig

 CONFIG_ARM=y
-CONFIG_ARCH_VERSATILE=y
 CONFIG_SYS_EXTRA_OPTIONS="ARCH_VERSATILE_PB"
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_BOOTD is not set

configs/versatileqemu_defconfig

 CONFIG_ARM=y
-CONFIG_ARCH_VERSATILE=y
 CONFIG_SYS_EXTRA_OPTIONS="ARCH_VERSATILE_QEMU,ARCH_VERSATILE_PB"
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_BOOTD is not set