kernel: differentiate board-specific components

Components like kernel, core, and bootstrap that are built for a
specific board need to reside inside the same architectural dependent
build directory. For instance there are sel4, foc, and hw kernel builds
for imx6q_sabrelite and imx7d_sabre, which have to reside inside the same
arm_v7 build directory.
This commit names those components explicitely, and adapts the run-tool to it.

Fix #3316

tool/builddir/build.mk

@@ -324,6 +324,7 @@ run/%: $(call select_from_repositories,run/%.run) $(RUN_ENV)
 	$(VERBOSE)$(GENODE_DIR)/tool/run/run --genode-dir $(GENODE_DIR) \
 	                                     --name $* \
 	                                     --specs "$(SPECS)" \
+	                                     --board "$(BOARD)" \
 	                                     --repositories "$(REPOSITORIES)" \
 	                                     --cross-dev-prefix "$(CROSS_DEV_PREFIX)" \
 	                                     --qemu-args "$(QEMU_OPT)" \