MFC: Merge more of the dtb machinery

Merge 278459,278460,278461,278462 which define DTBDIR and other things
needed for install to work. Although the commit in head kinda fixed
install_as_user, it's unknown if that works in 10.x (it didn't the last
time I tried).

MFC:

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

MFC: Merge more of the dtb machinery

Merge 278459,278460,278461,278462 which define DTBDIR and other things
needed for install to work. Although the commit in head kinda fixed
install_as_user, it's unknown if that works in 10.x (it didn't the last
time I tried).

MFC:

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