Added a new way to retrieve list of kernel available
that uses directory /etc/kernels/availables/{sabayon,sabayon-lts}.
If this directory is present is not used current logic for
retrieve the list of kernel.
Under /etc/kernels/avaiables/ directory files with 'lts' extension
are used for retrieve list of LTS kernels.
An user could return his kernel list just if he creates and
maintains his file in a similar format:
$# cat /etc/kernel/availables/mykernel
sys-kernel/linux-sabayon:4.9
sys-kernel/linux-mykernel:4.4
This logic now is used as an additional feature after retrieve
the existing kernels based on virtual/linux-binary or
virtual/linux-binary-lts.
4649660050