Add ability to preserve existing snoop log

Currently a stack restart or device reboot will overwrite existing snoop
logs when the stack restarts. This patch adds a new configuration file
parameter "BtSnoopSaveLog" that, if enabled, will cause an existing
snoop log to be renamed to "<snoop_file_name>.last" before a new snoop
log is created.

This will allow for easier debugging of crashes that involve a reboot or
stack restart.

Change-Id: I4f56204d31a6bf2e96058938e8616eb6dee7360b