busybox: fix login applet on selinux

busybox: fix login applet on selinux

Currently the system boots up, but is unusable because pressing enter
does not provide login with error:

  login: can't get SID for root

This is happenning, because login.c passes the Linux username directly
to get_default_context(), while libselinux expects an SELinux user
identity, causing the call to fail for users without a matching SELinux
name (e.g., root) and aborting login on SELinux-enabled systems.

Fixes: #19075
Upstream-Status: Submitted [https://lists.busybox.net/pipermail/busybox/2025-April/091407.html]
Signed-off-by: Petr Štetiar <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19080
Signed-off-by: Hauke Mehrtens <[email protected]>