The related code seems to have been useful a long time ago (the need to use
an RLE file and to reduce the number of colours to 16 has been dropped as of
syslinux version 3.62). So replace obsolete code with a simple cp command.
Signed-off-by: Wolfgang Schweer <wschweer@arcor.de>
Shift the extra_image helper function from boot-x86 to common.sh to
help.
Call extra_image with || true for now on boot-arm64 so we don't fail
builds straight away. Remove that || true later once we're happy
things work.