2) Secondly, maybe I want to enable selinux that it's installed but
not activated. For do that, I have to change and put "selinux=1" in
kernel parameters (following the http://wiki.debian.org/SELinux),
but in slug it's installed redboot and apex and I don't know how can
http://www.nslu2-linux.org/wiki/Debian/LVM has a description about how
to change the command line that APEX will pass to the kernel.
3) Thirdly, I want to know how backup _all_ the contents in the slug
and how can I restore it if the new kernel I compiled not works
This will make a backup of your flash:
cat /dev/mtdblock? > mtd-backup
Use tar or any other tool to make a backup of the USB disk.