grub: flip the bootsplash background 180 degrees

c-libreboot, and GNU Boot, is a 180 U-turn versus
current libreboot policy, as seen here:

https://libreboot.org/news/policy.html



Therefore, rotate the bootsplash pic 180 degrees, for
comical effect.

Signed-off-by: Leah Rowe <leah@libreboot.org>