Device trees commonly contain arrays of strings for compatible nodes.
We recently extended the "sifive,test0" node in a backwards-compatible
way, but QEMU didn't contain an FDT function to set 'compatible =
"sifive,test1", "sifive,test0";'.  I've converted over the code from the
ARM virt board that was doing something similar to be a helper function,
which I could then use for RISC-V as well.

I haven't tested the ARM change, but I have tested the RISC-V one.  It
appears to parse correctly in Linux, and a DTC treats it the same way as
it treats the string arrays it compiles -- specifically:

    $ cat test.dts
    /dts-v1/;

    / {
        string = "stringa";
        strings = "string1", "string2";
    };
    $ dtc -I dts test.dts -O dtb -o test.dtb
    $ dtc -I dtb test.dtb -O dts -o out.dts
    $ cat out.dts
    /dts-v1/;

    / {
            string = "stringa";
            strings = "string1\0string2";
    };

and

    $ qemu-system-riscv64 -m virt,dumpdtb=out.dtb ...
    $ dtc -I dtb test.dtb -O dts -o out.dts
    $ cat out.dts
    ...
            test@100000 {
                reg = <0x00 0x100000 0x00 0x1000>;
                compatible = "sifive,test1\0sifive,test0";
        };
    ...

Changes since v1 <20191107222500.8018-1-palmer@sifive.com>:

* This is now a multiple patch series.
* The hepler function has been added and used by the RISC-V virt board.
* The ARM virt board has been converted to use the helper function