install.sh: improve Generic code path + Gentoo + tmpfs install (#304)

Signed-off-by: Adel KARA SLIMANE <adel.ks@zegrapher.com>

linux-tkg-config/prepare

@@ -1385,6 +1385,9 @@ exit_cleanup() {
 
   # Remove RPM temporary files left
   rm -rf ${HOME}/.cache/linux-tkg-rpmbuild
+  if [ "$_use_tmpfs" = "true" ]; then
+    rm -rf "$_tmpfs_path/linux-tkg"
+  fi
 
   # Community patches removal in case of failure
   for _p in ${_community_patches[@]}; do