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

PKGBUILD

@@ -30,6 +30,11 @@ _ispkgbuild="true"
 source "$_where"/customization.cfg # load default configuration from file
 source "$_where"/linux-tkg-config/prepare
 
+if [ -e "$_EXT_CONFIG_PATH" ]; then
+  msg2 "External configuration file $_EXT_CONFIG_PATH will be used and will override customization.cfg values."
+  source "$_EXT_CONFIG_PATH"
+fi
+
 _tkg_initscript
 
 if [[ "$_sub" = rc* ]]; then