util/grub.d/00_header.in: Wire grub.cfg to use env_block when present

This patch extends the generated grub.cfg so that it can use the
external environment block when the variable env_block is defined.
During boot, if env_block is set, grub.cfg builds a device path for it,
exports the variable, and then loads its contents in addition to the
normal grubenv file.

When GRUB writes variables such as next_entry or saved_entry, the save
commands are changed to write into env_block if it is set, and to fall
back to the grubenv file otherwise. In this way the external environment
block is used automatically, and existing commands like savedefault or
save_env do not need to change.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

util/grub.d/00_header.in

@@ -46,6 +46,13 @@ cat << EOF
 if [ -s \$prefix/grubenv ]; then
   load_env
 fi
+
+if [ "\${env_block}" ] ; then
+  set env_block="(\${root})\${env_block}"
+  export env_block
+  load_env -f "\${env_block}"
+fi
+
 EOF
 if [ "x$GRUB_BUTTON_CMOS_ADDRESS" != "x" ]; then
     cat <<EOF
@@ -54,7 +61,11 @@ if cmostest $GRUB_BUTTON_CMOS_ADDRESS ; then
 elif [ "\${next_entry}" ] ; then
    set default="\${next_entry}"
    set next_entry=
-   save_env next_entry
+   if [ "\${env_block}" ] ; then
+     save_env -f "\${env_block}" next_entry
+   else
+     save_env next_entry
+   fi
    set boot_once=true
 else
    set default="${GRUB_DEFAULT}"
@@ -65,7 +76,11 @@ else
 if [ "\${next_entry}" ] ; then
    set default="\${next_entry}"
    set next_entry=
-   save_env next_entry
+   if [ "\${env_block}" ] ; then
+     save_env -f "\${env_block}" next_entry
+   else
+     save_env next_entry
+   fi
    set boot_once=true
 else
    set default="${GRUB_DEFAULT}"
@@ -93,7 +108,12 @@ fi
 function savedefault {
   if [ -z "\${boot_once}" ]; then
     saved_entry="\${chosen}"
-    save_env saved_entry
+    if [ "\${env_block}" ] ; then
+      save_env -f "\${env_block}" saved_entry
+    else
+      save_env saved_entry
+    fi
+
   fi
 }