Allow the recipe to set GENIMAGE_COMPRESSION to "gzip" or "xz". Other types are trivial to add support for if desired. Even if the class doesn't yet support a given scheme, the recipe itself can just define the proper GENIMAGE_COMPRESS_DEPENDS[foo] and
GENIMAGE_COMPRESS_CMD[foo].
Allow the recipe to set GENIMAGE_COMPRESSION to "gzip" or "xz". Other types are trivial to add support for if desired. Even if the class doesn't yet support a given scheme, the recipe itself can just define the proper GENIMAGE_COMPRESS_DEPENDS[foo] and GENIMAGE_COMPRESS_CMD[foo].