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/builddir/etc/build.conf.hw_zynq

@@ -4,4 +4,10 @@ REPOSITORIES += $(GENODE_DIR)/repos/base-hw
 ## Kernel-specific run tool configuration
 ##
 
-RUN_OPT = --include boot_dir/hw --include power_on/qemu --include log/qemu
+RUN_OPT = --include boot_dir/hw
+
+## Set RUN_OPT to exectute this on Qemu
+RUN_OPT += --include power_on/qemu --include log/qemu
+
+## Set RUN_OPT to execute this on actual hardware.
+#RUN_OPT += --include image/uboot