x86/build: Don't verify mtools configuration file for isoimage

x86/build: Don't verify mtools configuration file for isoimage

If mtools.conf is not generated before, 'make isoimage' could complain:

  Kernel: arch/x86/boot/bzImage is ready  (#597)
    GENIMAGE arch/x86/boot/image.iso
   *** Missing file: arch/x86/boot/mtools.conf
  arch/x86/boot/Makefile:144: recipe for target 'isoimage' failed

mtools.conf is not used for isoimage generation, so do not check it.

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>
Fixes: 4366d57af1 ("x86/build: Factor out fdimage/isoimage generation commands to standalone script")
Link: http://lkml.kernel.org/r/1512053480-8083-1-git-send-email-changbin.du@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>