By replacing the formerly hard-coded $(GENODE_DIR)/tool/depot/ by the
variable DEPOT_TOOL_DIR, the depot tools can be hosted outside the
Genode source tree, i.e., as part of the Goa tool.
This patch replaces the toy downloader (that merely copied files
locally, for testing) with the mechanics needed to download files from a
www server.
It also changes the use of GPG to use detached signatures.
Issue #2339
