bitbake.conf: don't append MACHINE_OVERRIDES to OVERRIDES when it's empty

bitbake.conf: don't append MACHINE_OVERRIDES to OVERRIDES when it's empty

* empty MACHINE_OVERRIDES adds :: to OVERRIDES (then in FILESPATH skips
  every directory after this)
* strip() is needed to remove space in case some distribution has ie
  MACHINE_OVERRIDES ?= ""
  and then
  MACHINE_OVERRIDES += ""
  or
  MACHINE_OVERRIDES ?= "${MACHINE_CLASS}", where MACHINE_CLASS is empty again

While FILESPATH was fixed separately, having :: in OVERRIDES still makes
bitbake 1.8.18 to throw a "RuntimeError: Set changed size during iteration"
exception when used together with collections.inc, since bitbake is being
re-spawned in there. More details here:

http://thread.gmane.org/gmane.comp.handhelds.openembedded/38254/focus=38595

Acked-by: Chase Maupin <chase.maupin@ti.com>
Signed-off-by: Denys Dmytriyenko <denis@denix.org>