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

repos/base-sel4/lib/mk/spec/imx7d_sabre/core-sel4.mk

@@ -1,13 +0,0 @@
-GEN_SRC_CC = platform_services.cc
-
-REP_SRC_CC = \
-             spec/arm/boot_info.cc \
-             spec/arm/irq.cc \
-             spec/arm/platform.cc \
-             spec/arm/platform_thread.cc \
-             spec/arm/thread.cc \
-             spec/arm/vm_space.cc
-
-INC_DIR += $(REP_DIR)/src/core/spec/arm
-
-include $(REP_DIR)/lib/mk/core-sel4.inc

repos/base-sel4/src/kernel/sel4/target.mk

@@ -1,5 +1,6 @@
-TARGET = sel4
-LIBS   = kernel-sel4
+BOARD ?= unknown
+TARGET = sel4-$(BOARD)
+LIBS   = kernel-sel4-$(BOARD)
 
 $(INSTALL_DIR)/$(TARGET):
-	$(VERBOSE)ln -sf $(LIB_CACHE_DIR)/kernel-sel4/kernel.elf.strip $@
+	$(VERBOSE)ln -sf $(LIB_CACHE_DIR)/kernel-sel4-$(BOARD)/kernel.elf.strip $@