bsdinstall: Ensure logging retained when BSDINSTALL_LOG is not changed by preamble

[stevek]

Summary

Before the preamble script is sourced, initialize BSDINSTALL_LOG with the
file in $debugFile, if it is not already set.

Without this change, the bsdinstall script would assume the preamble set
BSDINSTALL_LOG to empty and the comparison with $debugFile will fail,
causing the log to be re-initialized to /dev/null.

Test Plan

Found during use of scripted bsdinstall.
Without the change, all debug log output after the preamble is lost.
With the change, the log output is seen in the log file, as expected.
Setting BSDINSTALL_LOG in the preamble causes the bsdinstall script to
re-initialize the log to point to the appropriate place.

[dteske]

Thank for submitting. Give me a chance to review, but I do believe this looks like a good candidate for commit. Thank you