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.
- Make our test on arch/arm/config.mk be that the IMX_CONFIG file
  exists.

Signed-off-by: Tom Rini <trini@konsulko.com>