NVIDIA_DEVNET_MIRROR and SDK Manager

Jetpack 4.3 content as well as CUDA host tool support before this PR was not anonymously downloadable from NVIDIA's servers and requires an NVIDIA_DEVNET_MIRROR setup with the path to SDK manager downloads.

Attempting to build recipes which require host tool CUDA support will faied with message:

ERROR: Nothing PROVIDES 'cuda-binaries-ubuntu1804-native'
cuda-binaries-ubuntu1804-native was skipped: Recipe requires NVIDIA_DEVNET_MIRROR setup

To resolve, you must use the NVIDIA SDK manager to download the cotent to your build host, then add this setting to your build configuration (e.g., in conf/local.conf under your build directory):

NVIDIA_DEVNET_MIRROR = "file://path/to/downloads"

By default, the SDK Manager downloads to a directory called Downloads/nvidia/sdkm_downloads under your $HOME directory, so use that path in the above setting.

See example in tegra-demo-distro which demonstrates setting the path to the default download directory used by NVIDIA SDK manager.