imx: mkimage: add size check to the u-boot.imx make target

The make macro to check if the binary exceeds the board size limit is
taken straight from the root Makefile.

Without this and e.g. enabled EFI Vybrid fails booting as the regular
size limit check does not take the final u-boot.imx binary size into
account which is bigger due to alignment as well as IMX header stuff.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>