RFC: RTEMS Makefile change proposal

[Till Straumann]

I'm working often with different BSPs and even different
RTEMS releases for different projects.

Therefore, I created a small framework of 'site' specific
Makefiles for building RTEMS applications / libraries etc.

Putting this together, I came across the following issue:
Usually, RTEMS Makefile pieces are included in the order

   Makefile.inc    (defines RTEMS_BSP, exec_prefix, CC)
   $(RTEMS_CUSTOM)
   leaf / directory.cfg

RTEMS_CUSTOM normally is the bsp specific xxx.cfg which
in turn includes 'default.cfg'

All of the BSP configuration files xxx.cfg look like
this:

     include $(RTEMS_ROOT)/make/custom/default.cfg

     RTEMS_CPU = xxx
     RTEMS_CPU_MODEL = yyy
     RTEMS_BSP_FAMILY = zzz

PROPOSAL:
   I propose to define RTEMS_CPU etc. _before_ including
   'default.cfg'. This would make the CPU info available
   to 'default.cfg'. Hence, Makefile.inc (or default.cfg
   itself) could use an

     exec_prefix = $(prefix)/$(RTEMS_CPU)-rtems

   rather than hardcoding the CPU type (currently not
   possible because 'default.cfg' references 'exec_prefix'
   before RTEMS_CPU is defined).

RATIONALE:
   Making this change, one is able to use a generic,
   site-specific (more complex than the default; e.g.
   adds include/library paths + more) Makefile.inc which
   is usable across different BSPs and RTEMS releases etc.
   All the necessary information can be derived from the
   'prefix' and the 'RTEMS_BSP' variables which are
   the only ones to be set by the user.

If there are no objections, I'll put a patch together
and file a PR

-- Till