tools/bootconfig: Add --init option for bconf2ftrace.sh

Since the ftrace current setting may conflict with the new setting
from bootconfig, add the --init option to initialize ftrace before
setting for bconf2ftrace.sh.

E.g.
$ bconf2ftrace.sh --init boottrace.bconf

This initialization method copied from selftests/ftrace.

Link: https://lkml.kernel.org/r/159704853203.175360.17029578033994278231.stgit@devnote2

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>