bootctl: honor $KERNEL_INSTALL_CONF_ROOT

Honor $KERNEL_INSTALL_CONF_ROOT for reading config files, as
kernel-install does.

src/basic/path-util.h

@@ -130,6 +130,7 @@ int fsck_exists_for_fstype(const char *fstype);
 
 /* Similar to path_join(), but only works for two components, and only the first one may be NULL and returns
  * an alloca() buffer, or possibly a const pointer into the path parameter. */
+/* DEPRECATED: use path_join() instead */
 #define prefix_roota(root, path)                                        \
         ({                                                              \
                 const char* _path = (path), *_root = (root), *_ret;     \