Can't Build imx-hailo-demo-image with current instructions in README

[stevencnix]

Trying to build the demo image with the current instructions fails due to FFMPEG having a restricted license. Please update the instructions to account for this:

`
Parsing of 3215 .bb files complete (3213 cached, 2 parsed). 4772 targets, 287 skipped, 5 masked, 0 errors.
NOTE: Resolving any missing task queue dependencies
ERROR: Nothing RPROVIDES 'ffmpeg' (but /imx-yocto/sources/meta-solidrun-arm-imx8/recipes-core/images/imx-hailo-demo-image.bb RDEPENDS on or otherwise requires it)
ffmpeg was skipped: because it has a restricted license 'commercial'. Which is not listed in LICENSE_FLAGS_ACCEPTED
ffmpeg was skipped: because it has a restricted license 'commercial'. Which is not listed in LICENSE_FLAGS_ACCEPTED
NOTE: Runtime target 'ffmpeg' is unbuildable, removing...
Missing or unbuildable dependency chain was: ['ffmpeg']
ERROR: Required build target 'imx-hailo-demo-image' has no buildable providers.
Missing or unbuildable dependency chain was: ['imx-hailo-demo-image', 'ffmpeg']

Summary: There were 2 ERROR messages, returning a non-zero exit code.
`

[stevencnix]

The solution is to edit imx-yocto/build-fsl-imx-xwayland-imx8mpsolidrun/conf/local.conf and add the line

LICENSE_FLAGS_ACCEPTED = "commercial_ffmpeg"