dts: support building all dtb files for a specific vendor

dts: support building all dtb files for a specific vendor

This adjusts OF_UPSTREAM to behave more like the kernel by allowing for
all the devicetree files for a given vendor to be compiled. This is
useful for Qualcomm in particular as most boards are supported by a
single U-Boot build just provided with a different DT.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on Amlogic boards builds