hxtool: Split srst/erst add checks

Split the SRST/ERST case and add some checks.
This is mainly to make it easier to add some checks in following
patches.

Signed-off-by: Dr. David Alan Gilbert <dave@treblig.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20260504174914.122607-3-dave@treblig.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/hxtool

@@ -2,15 +2,29 @@
 
 hxtoh()
 {
-    flag=1
+    in_rst=0
     while read -r str; do
         case $str in
             HXCOMM*)
             ;;
-            SRST*|ERST*) flag=$(($flag^1))
+            SRST*)
+              if [ $in_rst -eq 1 ]
+              then
+                echo "Error: SRST inside another RST" >&2
+                exit 1
+              fi
+              in_rst=1
+            ;;
+            ERST*)
+              if [ $in_rst -eq 0 ]
+              then
+                echo "Error: ERST already outside RST" >&2
+                exit 1
+              fi
+              in_rst=0
             ;;
             *)
-            test $flag -eq 1 && printf "%s\n" "$str"
+            test $in_rst -eq 0 && printf "%s\n" "$str"
             ;;
         esac
     done