tools: mkimage: Check for datafile when type is script

If generating a script image and no datafile has been passed in, mkimage
dies with SIGSEGV:

  #0  __strchr_sse2 () at ../sysdeps/x86_64/multiarch/../strchr.S:32
  #1  0x0000000000403818 in main
      at tools/mkimage.c:503

Add explicit test for datafile to fix this.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>