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/riotboard_defconfig b/configs/riotboard_defconfig
index 498278a..0fc066d 100644
--- a/configs/riotboard_defconfig
+++ b/configs/riotboard_defconfig
@@ -7,6 +7,7 @@
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0x60000
+CONFIG_IMX_CONFIG="board/boundary/nitrogen6x/nitrogen6s1g.cfg"
 CONFIG_MX6S=y
 CONFIG_TARGET_EMBESTMX6BOARDS=y
 CONFIG_SYS_I2C_MXC_I2C1=y
@@ -20,7 +21,7 @@
 CONFIG_SPL=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_DISTRO_DEFAULTS=y
-CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/boundary/nitrogen6x/nitrogen6s1g.cfg,SPL,DDR_MB=1024"
+CONFIG_SYS_EXTRA_OPTIONS="SPL,DDR_MB=1024"
 CONFIG_BOOTCOMMAND="run finduuid; run distro_bootcmd"
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y