mplayer2: drop virtual/kernel dependency and s/ARM/arm/ in ARM_INSTRUCTION_SET

* virtual/kernel was needed only to provide omapfb.h in staging
  but that's now moved to BSP layers, no need to make sstate hash
  machine specific here.
* s/ARM/arm/ is just cosmetic change because classes are testing only
  '== "thumb"' so it works the same with ARM but mplayer2 was only
  recipe using it uppercase

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>