[mhwd-kernel] add rt-kernel support and some small modifications

I've tested this thorrowly running different kernels and all possible scenarios I could think of and everything seems to work fine.

kernel_repo()

kernel_list()

Some stuff is even a little fancier now 😉

The only little flaw is that when rt switches to a different basekernel this has to be adjusted manually until someone comes up with a more brilliant solution than this:

rt_base=linux46
rt_lts_base=linux44

IFS=. read -r major minor _ <<< "$(uname -r)"
current="linux$major$minor"
[[ $(uname -r) == *rt* ]] && [[ $current == $rt_base ]] && current=linux-rt-manjaro
[[ $(uname -r) == *rt* ]] && [[ $current == $rt_lts_base ]] && current=linux-rt-lts-manjaro