fix: accept underscores and hyphens in usernames

[tomatotomata]

What changed

Fix the username handling used by armbian-firstlogin so names such as armbian_user and armbian-user are accepted consistently with the issue report in armbian/configng#961.

The validation pattern now permits _ and - after the initial letter, and the normalization step preserves those characters instead of silently removing them before useradd runs. Names must still start with a letter and remain limited to ASCII letters, digits, _, and -.

The related report is in the armbian/configng tracker, but the affected script is maintained in this repository under packages/bsp/common/usr/lib/armbian/armbian-firstlogin.

Validation

• bash -n on the script after removing the repository file's existing CRLF line endings
• Bash checks for armbian_user, armbian-user, and User_2
• Rejection checks for _armbian and armbian.user
• git diff --check

The separate Imager-side validation mentioned in the report is intentionally out of scope for this focused build-side fix.

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