imx: Finish migration of IMX_CONFIG to Kconfig

- Provide a default Kconfig value of the default script
- Largely continue to define this via the board Kconfig file
- For the boards that select a script based on defconfig rather than
  TARGET, keep this within the defconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/imx28_xea_defconfig b/configs/imx28_xea_defconfig
index ba7eeb7..e17abfb 100644
--- a/configs/imx28_xea_defconfig
+++ b/configs/imx28_xea_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SYS_MALLOC_F_LEN=0x800
 CONFIG_NR_DRAM_BANKS=1
+CONFIG_IMX_CONFIG=""
 CONFIG_DM_GPIO=y
 CONFIG_SPL_DM_SPI=y
 CONFIG_DEFAULT_DEVICE_TREE="imx28-xea"