busybox: update login utils capabilities

[nemith]

When trying to automate user mangement on my tp2 bmc interfaces i found a lack of a couple functionalities that would make things much easier and less error prone.

The first is the CONFIG_CHPASSWD=y busybox config which add the chpasswd utility which allows changing user password from scripts much better than just passwd.

The other is CONFIG_FEATURE_ADDUSER_TO_GROUP busybox config option to update group memebership with the command adduser <user> <group>.

Also added the CONFIG_FEATURE_DEL_USER_FROM_GROUP option just as additional benefit, but not needed for my purposes.

Image size comparison

Before (master build)

vscode ➜ /src/buildroot (master) $ ls -l output/images/{tp2-bmc-firmware-sdcard.img,rootfs.erofs}
-rw-r--r--. 1 vscode vscode 37396480 Jul 23 18:25 output/images/rootfs.erofs
-rw-r--r--. 1 vscode vscode 55222272 Jul 23 18:27 output/images/tp2-bmc-firmware-sdcard.img

This branch

vscode ➜ /src/buildroot (nemith/busybox_login_utils) $ ls -l output/images/{tp2-bmc-firmware-sdcard.img,rootfs.erofs}
-rw-r--r--. 1 vscode vscode 37396480 Jul 23 18:31 output/images/rootfs.erofs
-rw-r--r--. 1 vscode vscode 55222272 Jul 23 18:32 output/images/tp2-bmc-firmware-sdcard.img

Seems to be identical but I am unfamiliar how these are packed. If there is a better way at judging the rom space increase please let me know.

Testing

# adduser bbennett wheel
# cat /etc/group | grep wheel
wheel:x:10:root,bbennett
# chpasswd --help
BusyBox v1.36.1 (2024-07-23 18:29:15 UTC) multi-call binary.

Usage: chpasswd [-me] [-c ALG] [-R DIR]

Read user:password from stdin and update /etc/passwd

	-e	Supplied passwords are in encrypted form
	-m	Encrypt using md5, not des
	-c ALG	des,md5,sha256/512 (default md5)
	-R DIR	Directory to chroot into
# chpasswd
bbennett:foo
chpasswd: password for 'bbennett' changed

[svenrademakers]

I strange that the images sizes are identical, my assumption is a issue with retriggering certain components. It would not be the first time. anyway, i approve