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

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.

Reviewed by:	dteske
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D25343

0 files changed, 0 insertions, 0 deletions