firstlogin: use useradd/groupadd instead of the adduser suite

Debian forky demoted adduser from the essential/minimal set, so minimal
images ship without it and armbian-firstlogin fails at user creation
(adduser: command not found) — reported in armbian/os#470 on Forky minimal.

Switch to the passwd (Essential) tools, matching a6cb68a (addgroup ->
groupadd in rockchip family_tweaks):
  - user creation: adduser --disabled-password --home ... --gecos ...
    -> useradd --create-home --home-dir ... --shell "$SHELL_PATH"
       --comment ... --user-group  (per-user group, skel copy, disabled
       password preserved; password is set right after as before)
  - docker group: addgroup --system --quiet -> groupadd --system
    (getent guard already present)
  - /etc/adduser.conf DSHELL edit now guarded on the file existing;
    useradd reads /etc/default/useradd, which set_shell already updates.

No new package needed; useradd/groupadd are Essential (passwd) so present
on every image including minimal.

Signed-off-by: Igor Pecovnik <igor@armbian.com>

Author: igorpecovnik

packages/bsp/common/usr/lib/armbian/armbian-firstlogin

@@ -258,9 +258,11 @@ set_shell() {
 	SHELL_PATH=$(grep "/$USER_SHELL$" /etc/shells | tail -1)
 	chsh -s "$(grep -iF "/$USER_SHELL" /etc/shells | tail -1)"
 
-	# change shell for future users
+	# change shell for future users. useradd (Essential, from passwd) reads
+	# /etc/default/useradd; /etc/adduser.conf only exists when the (now optional
+	# on Debian forky+) adduser package is installed, so guard that edit.
 	sed -i "s|^SHELL=.*|SHELL=${SHELL_PATH}|" /etc/default/useradd
-	sed -i "s|^DSHELL=.*|DSHELL=${SHELL_PATH}|" /etc/adduser.conf
+	[[ -f /etc/adduser.conf ]] && sed -i "s|^DSHELL=.*|DSHELL=${SHELL_PATH}|" /etc/adduser.conf
 
 }
 
@@ -586,7 +588,13 @@ add_user() {
 				RealName="$PRESET_DEFAULT_REALNAME"
 			fi
 
-			adduser --quiet --disabled-password --home /home/"$RealUserName" --gecos "$RealName" "$RealUserName"
+			# useradd (Essential, from passwd) instead of adduser (optional on
+			# Debian forky+, absent from minimal images). --user-group mirrors
+			# adduser's per-user group; --create-home copies /etc/skel; login shell
+			# comes from SHELL_PATH (set_shell ran first). Password is disabled here
+			# and set right below, same as adduser --disabled-password.
+			useradd --create-home --home-dir /home/"$RealUserName" --shell "$SHELL_PATH" \
+				--comment "$RealName" --user-group "$RealUserName"
 
 			# download and add SSH key if defined
 			if [[ -n "${PRESET_USER_KEY}" ]]; then
@@ -608,7 +616,7 @@ add_user() {
 			# (docker-ce package creates this group automatically during postinst, but we create it early
 			# to guarantee group membership is ready immediately after user creation.)
 			if ! getent group docker >/dev/null; then
-				if ! addgroup --system --quiet docker 2>/dev/null; then
+				if ! groupadd --system docker 2>/dev/null; then
 					echo "Warning: Failed to create docker group" >&2
 				fi
 			fi