Ensure Armbian system is always unmounted

[hboyd2003]

This pull request reworks the exit & error logic around the chroot command in recompile to ensure that the Armbian system is always unmounted before the script exits.

Improvements to exit & error logic:

• Encapsulated the logic for unmounting the Armbian system into unmount_armbian helper function.
• Integrated unmount_armbian before exiting with an error message when chroot fails.
• Added a trap for SIGINT (CTRL + C) that calls unmount_armbian.