linux: Allow running without chromeos-low_mem
linux: Allow running without chromeos-low_mem

Keep desktop builds running by making the cros-specific low memory
device optional.

BUG=none
TEST=Boot a kernel with crosvm on ubuntu desktop

Change-Id: I3c90a40e877fee8ed6763de027b0e25deec36373
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1157198
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Reviewed-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm