fix update-initramfs 'Exec format error'

Without "shebang" update-initramfs make an error.

Without => 
# update-initramfs -u
update-initramfs: Generating /boot/initrd.img-3.8.13.28
run-parts: failed to exec /etc/initramfs/post-update.d//flash-kernel: Exec format error
run-parts: /etc/initramfs/post-update.d//flash-kernel exited with return code 1
#

With => 
# update-initramfs -u
update-initramfs: Generating /boot/initrd.img-3.8.13.28
#