preunpackfs: Fix typo
extlinuxcfg: Fix typo
preunpackfs: Create destination only if it doesn't exist
extlinuxcfg: Prefix labels with "linux"
extlinuxcfg: Find the correct root device

installPath is just where the boot loader is installed, loop over the
partitions list and find the device mounted as / instead.
extlinux: Setup mbr and activate boot partition
extlinuxcfg: Concatenate kernel and initramfs

Don't use os.path.join, for some reason it doesn't work with
root_mount_point.