psplash: update to latest git revision and clean up

psplash: update to latest git revision and clean up

Update SRCREV to pick up:

c359546 Fix psplash-systemd failures
3c0a4f3 Remove generated psplash-poky-img.h

Also:
* set the unit type in psplash-start.service to "notify" to
  complete the psplash-systemd race fix
* remove the rest of the now unnecessary has_png logic bits
* change the generated image header destination to B instead of S
  since that now works after the recent makefile changes, and will
  avoid unnecessarily polluting the source tree

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>