MFC r262626, r262626, r264979, r265154, r265162, r265163, r267319, r270863,
    r270991, r271045:

  Enhancements related to compiling FDT data...

 - Use proper include path for dtc as well as cpp.
 - Call cpp with -P to avoid printing line markings.
 - Allow FDT_DTS_FILE to be a list, either in the makedtb target, or in a
   kernel config file.
 - Add the dts include directory to the -I list when doing arm builds.
 - Invoke make_dtb with MACHINE defined for cross building friendliness.
 - Allow the make_dtb script to work outside of a "make buildkernel"
   context by setting MACHINE from uname -m if it's not set already.
 - Use sh -c '...' to launch the dtb build scripts with env prepended,
   otherwise it tries to treat the env var stuff as a script file name.