grub2|loopback: use '@' bookended placeholders

...for consistency with syslinux config placeholders and improved
clarity of what text is a placeholder.

the old placeholders without the bookends are still replaced for
user configs for backwards compatibility.

the new ones are little used just at the moment but are expected to
become used much more in later commits.

Gbp-Dch: Short

scripts/build/binary_loopback_cfg

@@ -276,10 +276,27 @@ escape_for_sed() {
 	echo -n "$1" | perl -npe 's/\n/\\n/gm'
 }
 
+# Those without '@' markers are for backwards compatibility
 sed -i \
+    -e "s|@LINUX_LIVE@|$(escape_for_sed "${LINUX_LIVE}")|" \
+    -e "s|@LINUX_INSTALL@|$(escape_for_sed "${LINUX_INSTALL}")|" \
+    -e "s|@LINUX_ADVANCED_INSTALL@|$(escape_for_sed "${LINUX_ADVANCED_INSTALL}")|" \
+    -e "s|@MEMTEST@|$(escape_for_sed "${MEMTEST}")|" \
+    -e "s|@KERNEL_GI@|${KERNEL_GI}|" \
+    -e "s|@INITRD_GI@|${INITRD_GI}|" \
+    -e "s|@APPEND_GI@|${APPEND_GI}|" \
+    -e "s|@KERNEL_DI@|${KERNEL_DI}|" \
+    -e "s|@INITRD_DI@|${INITRD_DI}|" \
+    -e "s|@APPEND_DI@|${APPEND_DI}|" \
+    -e "s|@KERNEL_LIVE@|${KERNEL_LIVE}|" \
+    -e "s|@INITRD_LIVE@|${INITRD_LIVE}|" \
+    -e "s|@APPEND_LIVE@|${APPEND_LIVE}|" \
+    -e "s|@LB_BOOTAPPEND_INSTALL@|${LB_BOOTAPPEND_INSTALL}|" \
+    -e "s|@LB_BOOTAPPEND_LIVE_FAILSAFE@|${LB_BOOTAPPEND_LIVE_FAILSAFE}|" \
+    -e "s|@LB_BOOTAPPEND_LIVE@|${LB_BOOTAPPEND_LIVE}|" \
     -e "s|LINUX_LIVE|$(escape_for_sed "${LINUX_LIVE}")|" \
-    -e "s|LINUX_ADVANCED_INSTALL|$(escape_for_sed "${LINUX_ADVANCED_INSTALL}")|" \
     -e "s|LINUX_INSTALL|$(escape_for_sed "${LINUX_INSTALL}")|" \
+    -e "s|LINUX_ADVANCED_INSTALL|$(escape_for_sed "${LINUX_ADVANCED_INSTALL}")|" \
     -e "s|MEMTEST|$(escape_for_sed "${MEMTEST}")|" \
     -e "s|KERNEL_GI|${KERNEL_GI}|" \
     -e "s|INITRD_GI|${INITRD_GI}|" \

share/bootloaders/grub-pc/grub.cfg

@@ -28,31 +28,31 @@ insmod play
 play 960 440 1 0 4 440 1
 
 # Live boot
-LINUX_LIVE
+@LINUX_LIVE@
 
 # You can add more entries like this
 # menuentry "Alternate live boot" {
-# linux KERNEL_LIVE APPEND_LIVE custom options here
-# initrd INITRD_LIVE
+# linux @KERNEL_LIVE@ @APPEND_LIVE@ custom options here
+# initrd @INITRD_LIVE@
 # }
 # menuentry "Alternate graphical installer" {
-# linux KERNEL_GI APPEND_GI custom options here
-# initrd INITRD_GI
+# linux @KERNEL_GI@ @APPEND_GI@ custom options here
+# initrd @INITRD_GI@
 # }
 # menuentry "Alternate textual installer" {
-# linux KERNEL_DI APPEND_DI custom options here
-# initrd INITRD_DI
+# linux @KERNEL_DI@ @APPEND_DI@ custom options here
+# initrd @INITRD_DI@
 # }
 
 # Installer (if any)
-LINUX_INSTALL
+@LINUX_INSTALL@
 
 submenu 'Advanced options...' {
 
 # More installer entries (if any)
-LINUX_ADVANCED_INSTALL
+@LINUX_ADVANCED_INSTALL@
 
 # Memtest (if any)
-MEMTEST
+@MEMTEST@
 
 }