image_types_tegra.bbclass: use IMAGE_UBOOT variable
image_types_tegra.bbclass: use IMAGE_UBOOT variable

to specify the u-boot image, rather than hard-coding.

Signed-off-by: Matt Madison <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@madison.systems>

madisongh/meta-tegra,jackmitch/meta-tegra,jackmitch/meta-tegra,madisongh/meta-tegra,madisongh/meta-tegra,jackmitch/meta-tegra,madisongh/meta-tegra,madisongh/meta-tegra