Fix evaluation bugs in mkarchiso

Guard the call to `_mksignature()` in `_prepare_airootfs_image()` by an if statement.
Using the `&&` logic leads to `_prepare_airootfs_image()` evaluating to false if `$gpg_key` is not set.

Add `_msg_info()` calls to `_set_override()` which prevent the function from evaluating to false if no override is
being done. Additionally this is great for debugging purposes.

Add `_msg_info()` calls to `_read_profile()` (which is great for debugging purposes).

Fixes #81
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