jetson-xavier-nx-devkit-emmc missing in DTB files list in env-vars.ts

[vipulgupta2048]

Customer couldn't specify a DTB file for jetson-xavier-nx-devkit-emmc, it seems it's missing here in this list: https://github.com/balena-io/open-balena-api/blob/master/src/features/vars-schema/env-vars.ts#L294

If the board jetson-xavier-nx-devkit-emmc does support this feature, then we should add it and enable the feature to be used in the dashboard. We do need to check SV capability as to which SV version started supporting this device type if it even does so.

Pinging @acostach

[jellyfish-bot]

[vipulgupta2048] This has attached https://jel.ly.fish/235954ba-4340-4be7-9c77-1d0853c8150c

[acostach]

The Xavier boards do not have u-boot support, so custom device-trees cannot be configured on the Xavier line-up