xea: config: Use CONFIG_PREBOOT from Kconfig

The usage of the preboot feature is now controlled via a separate Kconfig
option - namely CONFIG_USE_PREBOOT.
It must be enabled for preboot code executing commands now defined in
CONFIG_PREBOOT (also moved to the Kconfig).

After defining both CONFIG_USE_PREBOOT and CONFIG_PREBOOT in
imx28_xea_defconfig the define of CONFIG_PREBOOT shall be removed from
xea.h as it is redundant.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
diff --git a/configs/imx28_xea_defconfig b/configs/imx28_xea_defconfig
index dc42e30..e418d8c 100644
--- a/configs/imx28_xea_defconfig
+++ b/configs/imx28_xea_defconfig
@@ -25,6 +25,8 @@
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyAMA0,115200n8"
+CONFIG_USE_PREBOOT=y
+CONFIG_PREBOOT="run prebootcmd"
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SPL_BOARD_INIT=y
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set
diff --git a/include/configs/xea.h b/include/configs/xea.h
index df05819..1207f75 100644
--- a/include/configs/xea.h
+++ b/include/configs/xea.h
@@ -47,7 +47,6 @@
 #define CONFIG_SYS_LOAD_ADDR	CONFIG_LOADADDR
 
 /* Extra Environment */
-#define CONFIG_PREBOOT		"run prebootcmd"
 #define CONFIG_HOSTNAME		"xea"
 
 #define CONFIG_EXTRA_ENV_SETTINGS					\