[syslinux] Support for btrfs multi-device - in meantime a useful error message

[james harvey]

Attempting to install syslinux on a multi-device (raid1) btrfs volume
gives the head-scratching error of:

=============================
/boot/syslinux is device /dev/sda1
extlinux: path /boot/syslinux doesn't match device /dev/sda1
=============================

extlinux/main.c::validate_device_btrfs()::line 1247 returns -1 along
with all of the other error conditions, causing this set of
contradictory messages, and giving no indication of what the problem
is.