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.
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