[util] add check_pkg()

lib/util.sh

@@ -341,6 +341,12 @@ check_base() {
     fi
 }
 
+# install a pkg in the live session if not installed
 inst_needed() {
     [[ ! $(pacman -Q $1 2>/dev/null) ]] && pacman -Sy --noconfirm $1
 }
+
+# install a pkg in the chroot if not installed
+check_pkg() {
+    arch_chroot "[[ ! $(pacman -Q $1 2>/dev/null) ]] && pacman -Sy --noconfirm $1" 2>$ERR
+}