icecast: add SPDX license info per homepage

Signed-off-by: Ian Leonard <antonlacon@gmail.com>
CONTRIBUTING.md: typo in PKG_LICENSE_FILES

Per r33861, it should be PKG_LICENSE and PKG_LICENSE_FILES used for licensing information.

Signed-off-by: Ian Leonard <antonlacon@gmail.com>
misc: correct typo in Makefiles for PKG_LICENSE_FILES

Signed-off-by: Ian Leonard <antonlacon@gmail.com>