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/create_builddir

@@ -61,6 +61,8 @@ GENODE_DIR ?= $(realpath $(dir $(MAKEFILE_LIST))/..)
 #
 BUILD_DIR ?= $(GENODE_DIR)/build/$(PLATFORM)
 
+SHELL := bash
+
 ifneq ($(wildcard $(BUILD_DIR)),)
 $(PLATFORM):: build_dir_already_exists
 build_dir_already_exists:
@@ -75,6 +77,18 @@ endif
 #
 GENODE_ABS_DIR := $(realpath $(shell echo $(GENODE_DIR)))
 
+#
+# Define absolute path to the contrib directory as written to the
+# 'etc/build.conf' file. We use 'abs_path' instead of 'realpath' because the
+# contrib directory may not exist when the build directory is created. In this
+# case, 'realpath' would return an empty string.
+#
+ifeq ($(CONTRIB_DIR),)
+CONTRIB_ABS_DIR := $$(GENODE_DIR)/contrib
+else
+CONTRIB_ABS_DIR := $(abspath $(shell echo $(CONTRIB_DIR)))
+endif
+
 #
 # Generic build-directory preparation rules
 #
@@ -87,6 +101,7 @@ $(BUILD_DIR)/etc:
 $(BUILD_DIR)/etc/build.conf:: $(BUILD_CONF_PLATFORM)
 	@echo "GENODE_DIR    = $(GENODE_ABS_DIR)" > $@
 	@echo 'BASE_DIR      = $$(GENODE_DIR)/repos/base' >> $@
+	@echo 'CONTRIB_DIR   = $(CONTRIB_ABS_DIR)' >> $@
 
 #
 # Supply -no-kvm argument to Qemu for kernels that are incompatible with KVM