Support setting FDT properties with optional values.

Fix a bug found and documented by Bartlomiej Sieka where the optional
value on "fdt set <path> <prop> [<val>]" wasn't optional.

=> fdt mknode / testnode
=> fdt print /testnode
testnode {
};
=> fdt set /testnode testprop
=> fdt print /testnode
testnode {
        testprop;
};

Signed-off-by: Gerald Van Baren <vanbaren@cideas.com>