Makefile.inc: Add support for staged builds.

- Allow the RTEMS_ROOT to be conditionally supplied. This
  can be a staging area before being moved to the final
  install prefix location.

- Update the default.cfg to use RTEMS_ROOT and to not rely on
  the exec_prefix so it's paths can be staged.

- Fix and add the needed configure subs.

Closes #3768

make/custom/default.cfg

@@ -5,11 +5,13 @@
 # Created by Jiri Gaisler, 16-03-97  (who is owed a debt of gratitude
 #   for the initial RTEMS autoconf support.  Thanks. --joel)
 
-include $(exec_prefix)/$(RTEMS_BSP)/make/target.cfg
+RTEMS_TARGET = $(RTEMS_CPU)-rtems$(RTEMS_API)
+
+include $(RTEMS_ROOT)/$(RTEMS_TARGET)/$(RTEMS_BSP)/make/target.cfg
 include $(RTEMS_SHARE)/make/host.cfg
 
 include $(RTEMS_ROOT)/make/main.cfg
-include $(exec_prefix)/$(RTEMS_BSP)/make/bsp.cfg
+include $(RTEMS_ROOT)/$(RTEMS_TARGET)/$(RTEMS_BSP)/make/bsp.cfg
 
 ## Target compiler config file, if any
 CONFIG.CC = $(RTEMS_SHARE)/make/compilers/gcc-target-default.cfg

make/main.cfg

@@ -16,9 +16,9 @@ default_target: all
 # but could be overridden in custom files.
 #
 
-PROJECT_RELEASE=$(exec_prefix)/$(RTEMS_BSP)
-PROJECT_BIN=$(PROJECT_ROOT)/bin
-PROJECT_INCLUDE=$(PROJECT_RELEASE)/lib/include
+PROJECT_RELEASE ?= $(exec_prefix)/$(RTEMS_BSP)
+PROJECT_BIN = $(PROJECT_ROOT)/bin
+PROJECT_INCLUDE = $(PROJECT_RELEASE)/lib/include
 PROJECT_TOOLS = $(PROJECT_RELEASE)/build-tools
 
 ## translate VARIANT into VARIANT_V
@@ -77,6 +77,6 @@ clean-generic:
 	-$(RM) -r $(CLEAN_ADDITIONS)
 endif
 
-.PHONY:	$(RECURSE_TARGETS) 
+.PHONY:	$(RECURSE_TARGETS)
 .PHONY: clean-generic
 .PHONY: distclean-generic