hw_zynq: refactor specs

This is a generalisation approach of the hw_zynq target. As the boards
typically use UART1 instead of UART0 (used by qemu), we have to
distinguish between those. Moreover, in general hw_zynq does not imply
zynq_qemu anymore, so that the support of particular boards can be
placed in third-party or community repositories (e.g. Genode world).

Fixes #1926

tool/create_builddir

@@ -252,6 +252,9 @@ hw_rpi::
 
 hw_zynq::
 	@echo "SPECS = genode hw_zynq" > $(BUILD_DIR)/etc/specs.conf
+	@echo "SPECS += perf_counter" >> $(BUILD_DIR)/etc/specs.conf
+	@echo "## By default, hw_zynq is executed on Qemu" >> $(BUILD_DIR)/etc/specs.conf
+	@echo "SPECS += zynq_qemu" >> $(BUILD_DIR)/etc/specs.conf
 
 hw_odroid_xu::
 	@echo "SPECS = genode hw_odroid_xu" > $(BUILD_DIR)/etc/specs.conf