cboot-t18x: fix PROVIDES setting
cboot-t18x: fix PROVIDES setting

to handle the case where PREFERRED_PROVIDER_virtual/bootloader might
not be defined.

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

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