Platform "inherit" support

[jwise]

It would be good to be able to produce both a snowy_s3 and snowy_dvt (and snowy_qemu...) without having to hand-write all of the stuff needed in those config.mk files right now. Best would be to be able to do something like:

$(call platform_clone,snowy_dvt,snowy_common)
$(call platform_add,snowy_dvt,snowy_bluetooth)

$(call platform_clone,snowy_qemu,snowy_common)
$(call platform_add,snowy_qemu,qemu_bluetooth)

[jwise]

I did something like this in 7a32d16.