grub2|loopback: move advanced installer entries under own submenu

this takes a step forward in moving towards the same updated layout as
with syslinux; here we get:

 - <live entries>
 - Start installer
 - Advanced install options...
    - <full set of install options>
 - Advanced options...
    - Memory Diagnostic Tool (memtest86[+])

note that this only affects the default menu. custom configs are not
affected by this change.

further steps to complete the move to the updated layout will follow
later.

"Advanced options..." should perhaps be renamed later.

Gbp-Dch: Short

scripts/build/binary_loopback_cfg

@@ -233,6 +233,8 @@ fi
 # Assembling debian-installer configuration
 if [ "${LB_DEBIAN_INSTALLER}" != "none" ]
 then
+	ENABLE_INSTALL_MENU="true"
+
 	KERNEL_DI="/install/vmlinuz"
 	INITRD_DI="/install/initrd.gz"
 	APPEND_DI="vga=normal ${LB_BOOTAPPEND_INSTALL} --- quiet"
@@ -262,6 +264,8 @@ then
 		Grub_install_entry "Automatic install in text mode" "auto" "${KERNEL_DI}" "${INITRD_DI}" "auto=true priority=critical ${APPEND_DI}" "a"
 		Grub_install_entry "Rescue system in text mode" "rescue" "${KERNEL_DI}" "${INITRD_DI}" "rescue/enable=true ${APPEND_DI}" "r"
 	fi
+else
+	ENABLE_INSTALL_MENU="false"
 fi
 
 # Assembling memtest configuration
@@ -296,6 +300,7 @@ sed -i \
     -e "s|@LB_BOOTAPPEND_INSTALL@|${LB_BOOTAPPEND_INSTALL}|" \
     -e "s|@LB_BOOTAPPEND_LIVE_FAILSAFE@|${LB_BOOTAPPEND_LIVE_FAILSAFE}|" \
     -e "s|@LB_BOOTAPPEND_LIVE@|${LB_BOOTAPPEND_LIVE}|" \
+    -e "s|@ENABLE_INSTALL_MENU@|${ENABLE_INSTALL_MENU}|" \
     -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}")|" \

share/bootloaders/grub-pc/grub.cfg

@@ -45,13 +45,20 @@ play 960 440 1 0 4 440 1
 # }
 
 # Installer (if any)
+if @ENABLE_INSTALL_MENU@; then
+
 @LINUX_INSTALL@
 
-submenu 'Advanced options...' --hotkey=a {
+submenu 'Advanced install options...' {
 
 # More installer entries (if any)
 @LINUX_ADVANCED_INSTALL@
 
+}
+fi
+
+submenu 'Advanced options...' --hotkey=a {
+
 # Memtest (if any)
 @MEMTEST@