tools/bootconfig: Fix wrong __VA_ARGS__ usage

tools/bootconfig: Fix wrong __VA_ARGS__ usage

Since printk() wrapper macro uses __VA_ARGS__ without "##" prefix, it causes
a build error if there is no variable arguments (e.g. only fmt is
specified.) To fix this error, use ##__VA_ARGS__ instead of __VAR_ARGS__.

Link: http://lkml.kernel.org/r/158108370130.2758.10893830923800978011.stgit@devnote2
Fixes: 950313ebf79c ("tools: bootconfig: Add bootconfig command")
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>