build: Improve Makefile.inc

Support relocated BSP installations by letting the user provide
RTEMS_ROOT.

Close #5170.

spec/build/bsps/makeinc.yml

@@ -9,12 +9,12 @@ content: |
   #
 
   RTEMS_API = ${__RTEMS_MAJOR__}
-
   RTEMS_CPU = ${ARCH}
   RTEMS_BSP = ${BSP_NAME}
+  RTEMS_ROOT ?= ${PREFIX}
 
-  prefix = ${PREFIX}
-  exec_prefix = $${prefix}/${ARCH}-rtems${__RTEMS_MAJOR__}
+  prefix = $$(RTEMS_ROOT)
+  exec_prefix = $$(prefix)/$$(RTEMS_CPU)-rtems$$(RTEMS_API)
 
   CC_FOR_TARGET = ${PROGRAM_PREFIX}gcc
   CXX_FOR_TARGET = ${PROGRAM_PREFIX}g++
@@ -43,7 +43,6 @@ content: |
   export SIZE
   export OBJCOPY
 
-  RTEMS_ROOT  ?= $$(prefix)
   PROJECT_ROOT = $$(RTEMS_ROOT)
   RTEMS_CUSTOM = $$(RTEMS_ROOT)/make/custom/$$(RTEMS_BSP).cfg
   RTEMS_SHARE  = $$(RTEMS_ROOT)/share/rtems$$(RTEMS_API)