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/arch/arm/config.mk b/arch/arm/config.mk
index 16c63e1..b684d8b 100644
--- a/arch/arm/config.mk
+++ b/arch/arm/config.mk
@@ -158,7 +158,8 @@
 OBJCOPYFLAGS += -j .efi_runtime -j .efi_runtime_rel
 endif
 
-ifneq ($(CONFIG_IMX_CONFIG),)
+ifdef CONFIG_MACH_IMX
+ifneq ($(CONFIG_IMX_CONFIG),"")
 ifdef CONFIG_SPL
 ifndef CONFIG_SPL_BUILD
 INPUTS-y += SPL
@@ -174,6 +175,7 @@
 INPUTS-y += u-boot.vyb
 endif
 endif
+endif
 
 EFI_LDS := elf_arm_efi.lds
 EFI_CRT0 := crt0_arm_efi.o