install.sh: unified linux git folder across kernel versions (#127)

* Prompt _distro if undefined when using "uninstall-help" in install.sh

* Unified linux source code git folder

* Update README

* Fix "defaulting to arch" by prompting distro earlier

* Move variable definitions at the correct position in script

* Remove unneeded Ubuntu dependency

* Fix typo

* RPM distros: Move only tkg related RPMs and leave the ~/rpmbuild folder as-is

* Fix defaulting to _distro=Arch when using ./install.sh config

* Use custom location for rpmbuild folder

* Fix _topdir for make bin-rpm

* Move rpmbuild folder cleanup to exit_cleanup function

* Revert folder naming in Arch specific NUKR=false cleanup

* Revert Void Linux changes

* Shallow clone a longer time window for stable releases

depth=1 had issues if the kernel release
is newer than the tag in the git repo

* Move functions to install.sh

* Fix cpuopts patch file naming

* Improve install.sh help message

README.md

@@ -76,8 +76,7 @@ cd void-packages
 If you have to restart the build for any reason, run `./xbps-src clean linux-tkg` first.
 
 ### Other linux distributions
-If your distro is not DEB or RPM based, `install.sh` script can clone the kernel tree, patch and edit a `.config` file from your current distro's 
-that is expected either at ``/boot/config-`uname -r`.config`` or ``/proc/config.gz`` (otherwise it won't work as-is)
+If your distro is not DEB or RPM based, `install.sh` script can clone the kernel tree in the `linux-src-git` folder, patch and edit a `.config` file from the one that your current distro uses. It is expected either at ``/boot/config-`uname -r`.config`` or ``/proc/config.gz`` (otherwise it won't work as-is).
 
 The command to do for that is:
 ```