tools/bootconfig: Add histogram syntax support to bconf2ftrace.sh

Add histogram syntax support to bconf2ftrace.sh script.

Link: https://lkml.kernel.org/r/162856128672.203126.8240335908303312607.stgit@devnote2



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