mx31pdk: Convert CONFIG_MX31 flag to use Kconfig.

Move CONFIG_MX31 from mx31pdk.h to mx31pdk_defconfig and introduce
necessary Kconfig changes as well.

Signed-off-by: Magnus Lilja <lilja.magnus@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/mach-imx/mx3/Kconfig b/arch/arm/mach-imx/mx3/Kconfig
new file mode 100644
index 0000000..bf6a1c6
--- /dev/null
+++ b/arch/arm/mach-imx/mx3/Kconfig
@@ -0,0 +1,20 @@
+if ARCH_MX31
+
+config MX31
+       bool
+       default y
+choice
+	prompt "MX31 board select"
+	optional
+
+config TARGET_MX31PDK
+	bool "Support the i.MX31 PDK board from Freescale/NXP"
+	select BOARD_LATE_INIT
+	select SUPPORT_SPL
+	select BOARD_EARLY_INIT_F
+
+endchoice
+
+source "board/freescale/mx31pdk/Kconfig"
+
+endif