bootconfig: Cleanup dummy headers in tools/bootconfig

Cleanup dummy headers in tools/bootconfig/include except
for tools/bootconfig/include/linux/bootconfig.h.
For this change, I use __KERNEL__ macro to split kernel
header #include and introduce xbc_alloc_mem() and
xbc_free_mem().

Link: https://lkml.kernel.org/r/163187299574.2366983.18371329724128746091.stgit@devnote2



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