x86/build: Add more generated files to the .gitignore file

Some of the files generated by the build process were not listed.
Signed-off-by: Changbin Du <changbin.du@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: yamada.masahiro@socionext.com
Link: http://lkml.kernel.org/r/1509939179-7556-5-git-send-email-changbin.du@intel.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/boot/.gitignore

@@ -7,3 +7,6 @@ zoffset.h
 setup
 setup.bin
 setup.elf
+fdimage
+mtools.conf
+image.iso