When no reverse dependencies are available, it's impossible to guess
the correct kernel tag. So, kernel_tag is None, which causes issue
when used in execve() (of course). So, handle the case trying to read
package configuration file containing the proper "uname -r" output required
to switch /usr/src/linux to the newly installed kernel using
"eselect kernel set".
Please note that this file (RELEASE_LEVEL) is quite new, and older
packages might have been shipped without it.
Also see bug #2227