change -h to -n because it is more "portable"

Add -h to the link command so that we replace the target file each time; this
should fix:

*** Failed target: .BEGIN
*** Failed command: ln -sf /usr/src/sys/external/gpl2/dts/dist/arch/arm/boot/dts

Add -h to the link command so that we replace the target file each time; this
should fix:

*** Failed target: .BEGIN
*** Failed command: ln -sf /usr/src/sys/external/gpl2/dts/dist/arch/arm/boot/dts dts/arm
*** Error code 1 (ignored)
ln: dts/arm/dts: Permission denied

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Add support for building overlay DT blobs specified in the kernel
config file like so:

makeoptions DTS_OVERLAYDIR="${THISARM}/conf"
makeoptions DTS_OVERLAYS="
my-cool-overlay.dts
"

Add support for building overlay DT blobs specified in the kernel
config file like so:

makeoptions DTS_OVERLAYDIR="${THISARM}/conf"
makeoptions DTS_OVERLAYS="
my-cool-overlay.dts
"

The result is my-cool-overlay.dtbo in the kernel compile directory.

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Compile dtbs with -@ so that fdtoverlay(1) et.al. can symbolically
reference phandles.

- Allow multiple .BEGIN targets
- Make their protection consistent

use -@ like bsd.klinks.mk

Use similar restrictions to bsd.klinks.mk for .BEGIN rules.

handle absolute $S paths; we want to add ../ if they are relative only.

Add more complexity because of the path mess in the dts files. Requested
by jmcneill@

Add support for building dts files from multiple paths, idea from christos

do dts depend processing in two passes, one for /include/ and one for #include

Move the dts rule to a separate file and add dependencies handling.