Firstlogin: run provision script if it does exists

[igorpecovnik]

Description

Small fixes + experimental feature. Need this for automation purposes. Can be later done inside https://github.com/x13-me/armbian-init

• fixing motd setting executing bit at wrong place
• when setting fixed address with 1st run, remove DHCP on all lan networks

Reference: armbian/configng#365

How Has This Been Tested?

• Made image and run with 1st config + provisioning script

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules