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/mccmon6_sd_defconfig b/configs/mccmon6_sd_defconfig
index 6039b95..a7fe6c1 100644
--- a/configs/mccmon6_sd_defconfig
+++ b/configs/mccmon6_sd_defconfig
@@ -6,6 +6,7 @@
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x20000
 CONFIG_ENV_SECT_SIZE=0x20000
+CONFIG_IMX_CONFIG="board/liebherr/mccmon6/mon6_imximage_sd.cfg"
 CONFIG_MX6QDL=y
 CONFIG_TARGET_MCCMON6=y
 CONFIG_DEFAULT_DEVICE_TREE="imx6q-mccmon6"
@@ -18,7 +19,6 @@
 CONFIG_FIT=y
 CONFIG_SPL_LOAD_FIT=y
 # CONFIG_USE_SPL_FIT_GENERATOR is not set
-CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/liebherr/mccmon6/mon6_imximage_sd.cfg"
 # CONFIG_USE_BOOTCOMMAND is not set
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_SPL_BOARD_INIT=y