root.sh: Check for parameters

This makes shellcheck more happy and the code more robust towards
changes.

root.sh

@@ -5,6 +5,20 @@
 # SPDX-License-Identifier: GPL-3.0+
 #
 
+# Parmeters passed in via image builder:
+# Mandatory
+: "${apt_sources:?'apt_sources' not set}"
+: "${distro:?'distro' not set}"
+: "${packages:?'packages' not set}"
+: "${pureos_board:?'pureos_board' not set}"
+
+# Optional
+: "${build_tarball:-}"
+: "${ci_pkgs:-}"
+: "${fdt_file:-}"
+: "${kernel_deb:-}"
+: "${patched:-}"
+: "${qemu:-}"
 
 # distro and basedir set by calling script 
 basedir="$1"
@@ -201,8 +215,11 @@ function setup_locale()
 function install_pkgs()
 {
     chroot "${basedir}" apt-get -y update
+    # Need to split package list
+    # shellcheck disable=SC2086
     if ! chroot "${basedir}" apt -y install ${packages}; then
 	echo "Package installation failed, this might help to diagnose the problem:"
+	# shellcheck disable=SC2086
 	chroot "${basedir}" apt -y -o Debug::pkgProblemResolver=yes install -y --simulate ${packages}
     fi
 }
@@ -210,6 +227,7 @@ function install_pkgs()
 
 function setup_apt()
 {
+    # shellcheck disable=SC2001
     echo "${apt_sources}" | sed 's/|/\n/g' > "${basedir}/etc/apt/sources.list"
     echo "Set sources list to:"
     cat "${basedir}/etc/apt/sources.list"