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/marsboard_defconfig b/configs/marsboard_defconfig
index 2c7906c..f41de87 100644
--- a/configs/marsboard_defconfig
+++ b/configs/marsboard_defconfig
@@ -5,6 +5,7 @@
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0xC0000
 CONFIG_ENV_SECT_SIZE=0x2000
+CONFIG_IMX_CONFIG="board/boundary/nitrogen6x/nitrogen6q.cfg"
 CONFIG_MX6Q=y
 CONFIG_TARGET_EMBESTMX6BOARDS=y
 CONFIG_SYS_I2C_MXC_I2C1=y
@@ -13,7 +14,7 @@
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="imx6q-marsboard"
 CONFIG_DISTRO_DEFAULTS=y
-CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/boundary/nitrogen6x/nitrogen6q.cfg,DDR_MB=1024"
+CONFIG_SYS_EXTRA_OPTIONS="DDR_MB=1024"
 CONFIG_BOOTCOMMAND="run finduuid; run distro_bootcmd"
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y