imx-base.inc: add boot container condition and wks dependency
Boot container provided by mainline U-Boot build can be used directly
int the result image file, and does not depends on the imx-boot
component provided by NXP.
Define variable to be used for conditional inheritance on the
imx-boot-container class when 'imx-boot-container' is present in
machine overrides.
Modify WKS dependency to imx-boot for mainline BSP built for aarch64 to
use imx-boot when derivative does not set the boot container
compatibility in its overrides.
