Don't require older RTEMS arch's to set RTEMS_BSP

This uses the old rules for RTEMS target architecture names to
work out what the value of RTEMS_BSP should be if it isn't set.
Targets had to be named RTEMS-$(RTEMS_BSP) in previous releases.
With this change out-of-tree target configuration files should
still work, but they can't be used to create sub-architectures
without first setting RTEMS_BSP in their CONFIG.Common.<arch>
file.

configure/os/CONFIG.Common.RTEMS

@@ -27,6 +27,10 @@ ifneq ($(CONFIG),$(TOP)/configure)
 -include $(TOP)/configure/CONFIG_SITE.Common.RTEMS
 endif
 
+#--------------------------------------------------
+# Set RTEMS_BSP from T_A if not already done
+RTEMS_BSP ?= $(subst RTEMS-,,$(T_A))
+
 #-------------------------------------------------------
 # Pick up the RTEMS tool/path definitions from the RTEMS BSP directory.
 include $(RTEMS_BASE)/$(RTEMS_TARGET_CPU)-rtems$(RTEMS_VERSION)/$(RTEMS_BSP)/Makefile.inc