/boot cannot be inside of your LVM. Like a 50MB /boot ext2 or ext3 partition is what you need. (Mine is 100MB for roundness)
if you are trying to use grub2-mkconfig from (I'm assuming a chroot environment) a livecd make sure your /sys and /proc are mounted before chrooting.
mount --bind /proc /mnt/whatever/proc
mount --bind /sys /mnt/whatever/sys
chroot /mnt/whatever bash -l
if you haven't been using this method, the '...' is for mounting any other partitions; ie. /boot, /home, /dev (mandatory), cp /etc/resolv.conf /mnt/whatever/etc/ (also mandatory), etc.