Added script which handles kernel upgrades.

This fixes #88 and #89.
Part of the code was taken from goranche's pull request #95 .
no :smile: 

if I edit my config.txt, and leave it with a "option=bla" without a newline at the end, that code might end up putting the "kernel= ..." in that last line

so I'd change the line to (didn't test it):

    echo -e "\nkernel=vmlinuz-\$version" >> \$config_file

that way the "kernel=..." line will always be a separate line, no matter the previous content of config.txt