ARM: zynq: drop "optional" from board select in favor of ZC702

One disadvantage of commit a26cd04920 (arch: Make board selection
choices optional) is that Kconfig could create such an insane
.config file that no board is selected.

Rip off the "optional" again in favor of ZC702 as the default
target.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/mach-zynq/Kconfig

@@ -10,7 +10,7 @@ config ZYNQ_CUSTOM_INIT
 
 choice
 	prompt "Xilinx Zynq board select"
-	optional
+	default TARGET_ZYNQ_ZC702
 
 config TARGET_ZYNQ_ZED
 	bool "Zynq ZedBoard"

configs/zynq_zc702_defconfig

@@ -1,6 +1,5 @@
 CONFIG_ARM=y
 CONFIG_ARCH_ZYNQ=y
-CONFIG_TARGET_ZYNQ_ZC702=y
 CONFIG_DEFAULT_DEVICE_TREE="zynq-zc702"
 # CONFIG_SYS_MALLOC_F is not set
 CONFIG_SPL=y