Convert CONFIG_SPL_EXT_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/cpu/armv7/am33xx/Kconfig b/arch/arm/cpu/armv7/am33xx/Kconfig
index dc51e9b..efbc219 100644
--- a/arch/arm/cpu/armv7/am33xx/Kconfig
+++ b/arch/arm/cpu/armv7/am33xx/Kconfig
@@ -1,4 +1,8 @@
 if AM43XX
+
+config SPL_EXT_SUPPORT
+	default y
+
 config TARGET_AM43XX_EVM
 	bool "Support am43xx_evm"
 	select TI_I2C_BOARD_DETECT
diff --git a/arch/arm/cpu/armv7/omap3/Kconfig b/arch/arm/cpu/armv7/omap3/Kconfig
index 99a25f9..85ea140 100644
--- a/arch/arm/cpu/armv7/omap3/Kconfig
+++ b/arch/arm/cpu/armv7/omap3/Kconfig
@@ -1,5 +1,8 @@
 if OMAP34XX
 
+config SPL_EXT_SUPPORT
+	default y
+
 choice
 	prompt "OMAP3 board select"
 	optional
diff --git a/arch/arm/cpu/armv7/omap4/Kconfig b/arch/arm/cpu/armv7/omap4/Kconfig
index 49adb8e..6a05625 100644
--- a/arch/arm/cpu/armv7/omap4/Kconfig
+++ b/arch/arm/cpu/armv7/omap4/Kconfig
@@ -1,5 +1,8 @@
 if OMAP44XX
 
+config SPL_EXT_SUPPORT
+	default y
+
 choice
 	prompt "OMAP4 board select"
 	optional
diff --git a/arch/arm/cpu/armv7/omap5/Kconfig b/arch/arm/cpu/armv7/omap5/Kconfig
index a8600b1..5460d7a 100644
--- a/arch/arm/cpu/armv7/omap5/Kconfig
+++ b/arch/arm/cpu/armv7/omap5/Kconfig
@@ -1,5 +1,8 @@
 if OMAP54XX
 
+config SPL_EXT_SUPPORT
+	default y
+
 choice
 	prompt "OMAP5 board select"
 	optional