memtest: better handle obsolete LB_MEMTEST value

available values currently are memtest86+|memtest86|none; "false" is
presumably handled for backwards compatibility

there is no need to handle this in individual scripts. the right place
to handle it is in Set_Defaults as now done

Gbp-Dch: Short
Closes: #952866

functions/defaults.sh

@@ -715,6 +715,9 @@ Set_defaults ()
 
 	# Setting memtest option
 	LB_MEMTEST="${LB_MEMTEST:-none}"
+	if [ "${LB_MEMTEST}" = "false" ]; then
+		LB_MEMTEST="none"
+	fi
 
 	# Setting loadlin option
 	case "${LB_MODE}" in

scripts/build/binary_memtest

@@ -24,7 +24,7 @@ Arguments "${@}"
 Read_conffiles config/all config/common config/bootstrap config/chroot config/binary config/source
 Set_defaults
 
-if [ "${LB_MEMTEST}" = "false" ] || [ "${LB_MEMTEST}" = "none" ] || [ "${LB_CHROOT_FILESYSTEM}" = "none" ]
+if [ "${LB_MEMTEST}" = "none" ] || [ "${LB_CHROOT_FILESYSTEM}" = "none" ]
 then
 	exit 0
 fi

scripts/build/source_debian

@@ -99,8 +99,7 @@ syslinux
 grub
 EOF
 
-		if [ "${LB_MEMTEST}" != "false" ] && [ "${LB_MEMTEST}" != "none" ]
-		then
+		if [ "${LB_MEMTEST}" != "none" ]; then
 			echo "${LB_MEMTEST}" >> source-selection.txt
 		fi
 		;;