imx6: bootscript-apalis: make it working on v2021.01 release

Upstream in commit 8b9c0cb46471 ("apalis_imx6: boot env configuration
updates") removed emmc legacy wrappers, but so far didn't included any
replacements. Fix it by simply defining the missing variables and UUID
gathering directly into the boot script.

Signed-off-by: Petr Štetiar <ynezz@true.cz>