grub: insert background in memdisk instead

the background is only a few kb. the whole rationale
before was to limit the space used in memdisk, but this
decision was made when the background was much bigger;
it has since been optimised greatly, and the grub modules
were heavily reduce, so it should be safe.

grub's memdisk breaks when you add too much data to it.
as part of simplifying the rest of lbmk, this change removes
some more bloat from the rest of lbmk. handling this in the
memdisk is much simpler than handling it with cbfstool.

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

script/trees

@@ -286,7 +286,8 @@ mkpayload_grub()
 	    --fonts= --themes= --locales=  --modules="$grub_modules" \
 	    --install-modules="$grub_install_modules" \
 	    "/boot/grub/grub_default.cfg=${cdir}/.config" \
-	    "/boot/grub/grub.cfg=$grubdata/memdisk.cfg" || \
+	    "/boot/grub/grub.cfg=$grubdata/memdisk.cfg" \
+	    "/background.png=$grubdata/background/background1280x800.png" || \
 	    $err "$tree: cannot build grub.elf"
 }