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/mx28evk_defconfig b/configs/mx28evk_defconfig
index 8c4f1a2..1ed7c0c 100644
--- a/configs/mx28evk_defconfig
+++ b/configs/mx28evk_defconfig
@@ -7,6 +7,7 @@
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x4000
 CONFIG_ENV_OFFSET=0x40000
+CONFIG_IMX_CONFIG=""
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="imx28-evk"
 CONFIG_SPL_TEXT_BASE=0x00001000