vectorscope_no_soc: cache optional, default off #435

Job	Run time
ubuntu-bitstream-dsp-nco	1m 57s
ubuntu-bitstream-dsp-midicv	1m 3s
ubuntu-bitstream-boot-stub	56s
ubuntu-bitstream-boot	4m 47s
ubuntu-bitstream-poly	7m 50s
ubuntu-bitstream-selftest	4m 55s
ubuntu-bitstream-vectorscope	1m 38s
ubuntu-bitstream-xbeam	4m 56s
ubuntu-sim-dsp-nco	53s
ubuntu-usb-audio	1m 40s
unit-tests	39s
	31m 14s

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