arm-trusted-firmware-mvebu: stop cluttering Image Builder

arm-trusted-firmware-mvebu: stop cluttering Image Builder

All contents of staging_dir/image are included in Image Builder (IB) in
case some binary needs to be included in final image. But in case of
this package, all sources are stored there and those clutter the final
tarball of IB for no reason. Those sources are not used during image
creation and are just dead weight. To put it in perspective, the IB for
21.02.0 is 158 MiB, 22.03.0-rc6 is 366 MiB and snapshot is over 620 MiB!
To fix it, put them in package build directory, so they won't end up
included in IB tarball.

Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>
Reviewed-by: Andre Heider <a.heider@gmail.com>