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/zmx25_defconfig b/configs/zmx25_defconfig
index 78277ce..f459c05 100644
--- a/configs/zmx25_defconfig
+++ b/configs/zmx25_defconfig
@@ -5,6 +5,7 @@
 CONFIG_ENV_SIZE=0x20000
 CONFIG_ENV_SECT_SIZE=0x20000
 CONFIG_TARGET_ZMX25=y
+CONFIG_IMX_CONFIG=""
 CONFIG_SYS_LOAD_ADDR=0x80000000
 CONFIG_BOOTDELAY=5
 CONFIG_AUTOBOOT_KEYED=y