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/bg0900_defconfig b/configs/bg0900_defconfig
index ec8ffdb..ca771ab 100644
--- a/configs/bg0900_defconfig
+++ b/configs/bg0900_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x4000
+CONFIG_IMX_CONFIG=""
 CONFIG_SPL_TEXT_BASE=0x00001000
 CONFIG_TARGET_BG0900=y
 CONFIG_SPL_SERIAL_SUPPORT=y