Build-system support for ported 3rd-party code

The new 'select_from_ports' function allows a target description file to
query the path to an installed port. All ports are stored in a central
location specified as CONTRIB_DIR. By default, CONTRIB_DIR is defined
as '<genode-dir>/contrib'. Ports of 3rd-party source code are managed
using the tools at '<genode-dir>/tool/ports/'.

Issue #1082
tool/prepare.mk: Switch directories silently
Migrate libports to new ports-management mechanism

Issue #1082
fixup libports (gmp)
fixup libports (icu)
fixup libports (jbig2dec)
fixup libports (dummy hash values)
fixup libports (jpeg)
fixup libports (libav)