MFC r279824, r279827, r279825:

Add a dtb module for AM335x systems (just Beaglebone right now).

Remove the static DTB config and instead build modules/dtb/am335x.

Also, remove WITHOUT_MODULES="ahc" which was added long ago to work
around build problems that have long since been fixed correctly.

Revert accidentally commited modules/Makefile file from r279824.

MFC r278338, r278340, r278458, r278519:

Create a module to install the Raspberry Pi dtb files.

Pull in the rpi.dts -> rpi.dtb module (dtb/rpi) and have it install
rpi.dtb in /boot/dtb by default.

Add a module to build the dtb files for all supported imx6 systems.

Remove imx6s-wandboard.dts, there is no such file.

MFC:

Merge r276846: Add infrastructure to build dtb files from dts files.

Add infrastructure to build dtb files from dts files.