Fix kernel selection after introducing $BASE/environments

After commit 3399f4b, ZBM no longer allows user to select
a kernel to boot, with the log error message:

ZFSBootMenu: kernel file zfsbootmenu/<pool and bootenv here>/kernels missing

That is due to that the path to the kernels list hasn't been updated
to use be_location() and still uses the old "zfsbootmenu/<pool>"
scheme. Doing `ln -s` from the recovery shell solves the problem.
Adjust that place accordingly to fix the issue.

Fixes: 3399f4b ("Move dataset mountpoint to $BASE/environments/")
Signed-off-by: Alexander Lobakin <alobakin@pm.me>

zfsbootmenu/lib/zfsbootmenu-lib.sh

@@ -135,7 +135,7 @@ draw_kernel() {
     return 130
   fi
 
-  _kernels="${BASE}/${benv}/kernels"
+  _kernels="$( be_location "${benv}" )/kernels"
   if [ ! -r "${_kernels}" ] ; then
     zerror "kernel file ${_kernels} missing"
     return 130