bootconfig: Support O=<builddir> option

Support O=<builddir> option to build bootconfig tool in
the other directory. As same as other tools, if you specify
O=<builddir>, bootconfig command is build under <builddir>.

Link: http://lkml.kernel.org/r/158323468033.10560.14661631369326294355.stgit@devnote2

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