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.

MFC 262614, 262625, 262626, 262627, 262682, 262714, 262725, 262736

Integrate device-tree upstream files into the build process:
(1) Invoke cpp to bring in files via #include (although the old
/include/ stuff is supported still).
(2) bring in files from either vendor tree or freebsd-custom files
when building.
(3) move all dts* files from sys/boot/fdt/dts to
sys/boot/fdt/dts/${MACHINE} as appropriate.
(4) encode all the magic to do the build in sys/tools/fdt/make_dtb.sh
so that the different places in the tree use the exact same logic.
(5) switch back to gpl dtc by default. the bsdl one in the tree has
significant issues not easily addressed by those unfamiliar with
the code.

Only try to build the static dtb when we're building a static dtb.

Use proper include path for dtc as well as cpp.

Fix syntax errors (missing ; other minor glitches) in existing dts files.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Flattened Device Tree helper scripts.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation