kernel: Eliminate duplication of #define FPU

To avoid conflicts with the definition of "FPU" by the external
cmsis sub-system, the symbol "FPU_LEGACY" is now used to denote
the FPU task group. (That is, the group of tasks that utilize
the CPU's floating point registers.)

Backwards compatibility impacts:

* Microkernel applications that define private tasks using the
  DEFINE_TASK() macro must now use "FPU_LEGACY" to indicate
  that the task is part of the FPU task group, rather than "FPU".

* Applications that perform operations on the members of the
  FPU task group must now use "FPU_LEGACY", rather than "FPU".
  (Note: There is no apparent reason for an application to
  perform such an operation, so this is unlikely to impact
  existing applications.)

Backwards compatibility non-impacts:

* Application MDEFs can continue to specify "FPU" when defining
  a public task that is part of the FPU task group.

* Applications written for the unified kernel are unaffected,
  since the kernel implements task groups differently.

Change-Id: I972eecbc7b50d66f0a4f095d2d5177b5ce90cb71
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>