Apply shellcheck recommendations (#1389)

* compile.sh: Apply shellcheck recommendations

* lib/upgrade.sh: Apply shellcheck recommendations

* lib/main.sh: Apply shellcheck recommendations

* lib/desktop.sh: Apply shellcheck recommendations

* lib/makeboarddeb.sh: Apply shellcheck recommendations

lib/upgrade.sh

@@ -1,3 +1,5 @@
+#!/bin/bash
+
 # Copyright (c) 2015-2017 Igor Pecovnik, igor.pecovnik@gma**.com
 # Copyright (c) 2015-2017 other Armbian contributors (https://github.com/armbian/build/graphs/contributors)
 #
@@ -13,12 +15,12 @@
 TTY_X=$(($(stty size | awk '{print $2}')-6)) # determine terminal width
 TTY_Y=$(($(stty size | awk '{print $1}')-6)) # determine terminal height
 
-if [[ $(basename $0) == main.sh ]]; then
+if [[ $(basename "$0") == main.sh ]]; then
 	echo "Please use compile.sh to start the build process"
-	exit -1
+	exit 255
 fi
 
-if [[ $(basename $0) == compile.sh ]]; then
+if [[ $(basename "$0") == compile.sh ]]; then
 	# assuming dialog was installed already since this is supposed to be shown only on upgrade from previous versions
 	dialog --title "Directory structure change notice" --colors --msgbox "Build script directory structure was changed to simplify the build environment setup, \
 	simplify upgrading in the future and improve compatibility with containers like Docker and Vagrant
@@ -28,5 +30,5 @@ if [[ $(basename $0) == compile.sh ]]; then
 	Default build configuration is defined in \Z1config-default.conf\Zn (created on first build script run)
 
 	Sorry for the inconvenience" $TTY_Y $TTY_X
-	exit -1
+	exit 255
 fi