d-l: splashscreen requires addition settings since 1.0~a32-1

[jpryzby+d at quoininc.com (Justin Pryzby)]

It used to be the case (and is still documented in /u/s/d/l-h/FAQ)
that putting a splash.rle in config/binary_syslinux/ would effect a
custom splashscreen, but this is no longer the case.  Not only do you
have to specifically set LH_SYSLINUX_SPLASH, you also have to use a
PNG (otherwise I think the rle file gets renamed to .png and thusly
syslinux doesn't display it).  I'm not sure if these are intended
consequences though.

Justin