If this switch is provided, kernel-switcher tries to look for
reverse dependencies of the currently running kernel (uname -r)
during the selection of external package modules that require to
be installed for the new kernel.
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
