tools/bootconfig: Fix to check the write failure correctly

Fix to check the write(2) failure including partial write
correctly and try to rollback the partial write, because
if there is no BOOTCONFIG_MAGIC string, we can not remove it.

Link: https://lkml.kernel.org/r/160576521135.320071.3883101436675969998.stgit@devnote2

Fixes: 85c46b78

 ("bootconfig: Add bootconfig magic word for indicating bootconfig explicitly")
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>